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  • Why are there no diesel pushrod engines?
  • whatgoesup
    Full Member

    I’ve got a degree in Aeronautical Engineering, I’ll survive.

    Perfect a then dive straight into the more “university lecture” stuff.

    Once up to speed the force is zero.

    Apart from the camshaft the valve train is rarely at constant speed – is is continually accelerating in one direction and then the other to drive the valve from its static “closed” position to moving quickly toward “open” and then decelerating to achieve its “open” position, then the reverse to close.

    So you’re saying mass has a direct influence on the force needed for a given accelaration?

    Yes. Apologies for the overly simple explanation earlier. Basically F=m.a

    thols2
    Free Member

    Well yes but that doesn’t help the case for pushrods though. The only reason they still have pushrods are because they have to. They also have to have carburettors too.

    Nascar engines use electronic fuel injection.

    The point about Nascar engines is that pushrod engines can rev much harder than people assume and can generate pretty decent horsepower.

    squirrelking
    Free Member

    The point about NASCAR engines is they have been developed within a prescribed et of parameters.

    Push rods are shit and belong in the dark ages outside of industrial applications. It’s operationally advantageous to be able to pull a cylinder head at sea with nothing more than a windy gun and a chain block, you pay for that in energy losses. Only medium speed engines use push rods last I checked since slow speed are entirely electronic and high speed use camshafts and belts.

    thols2
    Free Member

    high speed use camshafts and belts.

    I think you’ll find proper race engines use gear drives, not timing belts or chains. Timing belts are cheap and quiet, but aren’t up to the demands of proper high-performance engines.

    pondo
    Full Member

    The 1994 Indy 500 was won by a car with a pushrod engine, build buy Ilmor and funded by Mercedes-Benz.

    Exploiting a loophole that allowed 3.4 litre pushrod engines to run higher boost than their 2.6 litre competitors, right?

    thols2
    Free Member

    Exploiting a loophole that allowed 3.4 litre pushrod engines to run higher boost than their 2.6 litre competitors, right?

    Yes. It happened because they allowed stock-block engines with pushrods as a cheaper alternative to custom built race engines. Buick V6s were pretty common back in the 80s, but weren’t reliable because of the requirement to use stock parts. The stock parts requirement was lifted to help the Buick based engines. When Ilmor realized that the requirement for stock part had been removed, they built a custom engine to take advantage.

    https://www.hemmings.com/stories/2019/07/26/when-big-boosted-buicks-ruled-the-brickyard-scott-braytons-95-and-96-lola-t95-menards

    My point isn’t that pushrods perform as well as DOHC for racing engines, it’s that the idea that pushrod engines are “shit” isn’t quite right. They can rev a lot harder and put out more power than people often assume and the Chevy small-block engines seem to go pretty well. In the Australian V8 touring cars, the quad-cam engines struggled against the Nascar based pushrod engines because they were all rev limited to 7500 RPM and had limitations on camshafts to prevent an expensive engine development arms race. There was no power advantage to quad-cam engines under those regulations, but they used more fuel. The rules were relaxed a few years ago to allow turbo V6 engines in. GM built a quad-cam V6 turbo, but never raced it.

    kenneththecurtain
    Free Member

    This really is peak STW. Some dude who clearly knows about this stuff shows up and answers the OP’s question, then a bunch of other people show up and try and pick holes in the explanation.

    Love it.

    trail_rat
    Free Member

    I think you’ll find proper race engines use gear drives,

    If you put your part quote back. In context SK is clearly talking about industrial/boat applications as is his vocation.

    Main reason that we don’t use gear drives and pushrods outside of the racing world is that over the years we have grown to enjoy not driving with ear defenders on.

    wobbliscott
    Free Member

    The point about Nascar engines is that pushrod engines can rev much harder than people assume and can generate pretty decent horsepower.

    Yes I acknowledged that but the point was it’s not a valid example because it’s not the optimal solution. Free from the restriction of the rules of the game the engine designers would ditch pushrods and have a more technically optimal design.

    NASCAR is steeped in history and nostalgia which is hampering the innovation in the sport and channeling it into a single technical solution which they’re doing their best to ring the next out of.

    wobbliscott
    Free Member

    The future is camless and pneumatically actuated.
    Why pneumatic specifically? Is it a speed thing?

    Don’t know. It’s what F1 engines use and the Kenisegg that has a cam less valve train. I guess hydraulics would be too heavy and I think the compressibility of air might be a factor.

    bazzer
    Free Member

    Still no answer as to why push rod engines intrinsically have better bottom end power @trail_rat

    There are/were plenty of 2 valve OHC engines as anyone who drove an early 90s Sierra or Escort would attest.

    Obviously there are some cam profile and rpm limits to push rod engines without spending a lot of money on them so at a disadvantage over OHC. If you wanted a fatter bottom end on an OHC engine there is nothing stopping you having smaller valves and less overlap to get this (you could do this with pushrod too). You can also have bigger valves with more overlap and use higher revs, harder to do with a push rod engine.

    Force on the pushrod will go up with engine speed and how steep the opening profile of the cam lobe is. You effectively have a small (on a car engine) tube in compression so its not an ideal engineering solution if you want aggressive cams and lots of revs.

    trail_rat
    Free Member

    Still no answer as to why push rod engines intrinsically have better bottom end power @trail_rat

    I said good not better. Good by virtue of the missing top end…. If they were all mega bucks race engines with exotic internals and could have a full rev range then the statement would be …. Good range

    trail_rat
    Free Member

    And you hit the crux of the whole matter.

    Ohc engines are easier to tune to the characteristics you desire instead of living with compromises.

    bazzer
    Free Member

    I said good not better. Good by virtue of the missing top end…. If they were all mega bucks race engines with exotic internals and could have a full rev range then the statement would be …. Good range

    But there were also a lot of race escort engines running big valves and aggressive cams with lots of overlap that don’t have a great bottom end too.

    Basically there is nothing about a push rod engine that intrinsically ensures a good bottom end. In fact if you take most push rod engines that were main stream in the UK they didn’t have power anywhere 🙂

    I am not trying to give you a hard time, just trying to make sure people understand and don’t perpetuate myths.

    K
    Full Member

    Why are we still arguing about who has a better rock that their dads gave them to smash a nut, when there are nut crackers.

    thisisnotaspoon
    Full Member

    That is really easy to answer. See my first post for the main points. But… specifically relating to the valves and cams it goes like this.

    I know the limitations of pushrod engines. I’ve got one in my 2nd car, and it’s no longer possible to get properly hardened camshafts for it which makes the weaknesses accutely apparent when ever you measure the valve clearances.

    My point was specifically related to diesels. Which neither rev as quickly, nor rely so much on scavenging and the effects of momentum on the intake to the same extent due to forced induction. Even what Google suggests is the sportiest diesel (Audi S4 apparently) hits peak power (not torque, power) at 4000 rpm which is nowhere near troubling a pushrod.

    Pushrods, for all their limitations, would appear to be good enough that they could have been used (and were in some cases). The reason for their dissaperance seems much more to do with subsequent diesels being derived from petrol blocks and diesels popularity not really surging untill those petrol engines moving over to OHC layouts.

    wobbliscott
    Free Member

    Pushrods, for all their limitations, would appear to be good enough that they could have been used (and were in some cases).

    Good enough is no longer good enough. In Europe where we’ve not been building huge inefficient engine for decades, the push for more efficient engines had pushed manufacturers to build better and more efficient engines. Pushrods were one of the first things to go. The US has lagged behind that efficiency curve…they just solved the issue of power and torque by adding CC’s – hence much larger engines and the need for Lower profile cylinder heads – a kind of self fulfilling prophecy. The Europeans had no such luxury. And the influence on F1 vs NASCAR also helps. Europeans like peaky, reviver engines for the narrow and twisty mountain roads or country lanes, the yanks like to lollop along the freeway at 60mph with a big lazy V8 up front ticking over at 750 rpm and getting about 10mpg…but who gives a toss when fuel is cheaper than water.

    The reason for their dissaperance seems much more to do with subsequent diesels being derived from petrol blocks and diesels popularity not really surging untill those petrol engines moving over to OHC layouts.

    Well that and just simply better performance overall..what are the downsides to OHC? You say that as if production efficiency, supply chain efficiency, scaleable designs that can be used for many applications is not an important factor for an engine producer and that it is a bad thing. It’s already been confirmed by someone who does this for a living that pushrod engines, for cars at least, are limiting in design and performance, expensive to produce, have additional parts count and complexity and hamper design options. Again….exactly what are the benefits to pushrod engines apart from nostalgia and harking back to yesteryear? it is old tech and like any old tech, apart from the odd niche application, it’s been superseded by better technology.

    bazzer
    Free Member

    I would say its cost the main reason not used in pretty much any engine these days.

    OHC engines are cheaper to build now, would be my guess.

    whatgoesup
    Full Member

    Pushrods, for all their limitations, would appear to be good enough that they could have been used

    Pushrods are NOT “good enough” for modern diesels, hence why they’ve now been replaced in the vast majority of cases.

    peak power (not torque, power) at 4000 rpm which is nowhere near troubling a pushrod.

    If peak power is at 4000 rpm redline is likely circa 4,500 and the max over speed condition designed for will likely be a bit over 5,000.

    Pushrods don’t have a max rpm limit as such, it’s just that they’re overall higher mass hence limits are more challenging and you have to make greater design compromises in other areas (valve motion) to allow for this, which hurts performance and emissions.

    Although a diesel doesn’t rev to the same high rpm as a petrol it’s still fast enough for the valvetrain dynamics to be a limiting factor, so a lighter valvetrain allows for better performance and emissions.

    For comparison the 1500 and 1800RPM Diesel and Natural has engines we develop at work are still valvetrain dynamics limited and we design around these operating speeds and then work backwards to optimise valve motion (I.e. cam profiles) to the force limits that result from this operation. If we could use OHC on those we would ease these restrictions further and be able to have more freedom in cam profile design (we can’t because our engines need pushrods for the reasons I have in my first post)

    wobbliscott
    Free Member

    Any technology has to ‘win’ its way onto any product. It has to demonstrate its contribution to achieving the overall specification that drives the design. If there are no net benefits that satisfy all of the design specification then it doesn’t make its way onto a product. Its as simple as that.

    Peoples perception that engineers design the products they want to design is not true. They engineer the products that the market wants and is driven by very specific parameters driven by market desires (looks, performance, cost to buy and maintain, reliability etc), economics of production, and more and more dominating these days are political and regulatory requirements. So with all these competing requirements any technology that can’t justify its inclusion by ticking all of these boxes has no right to be selected….hence the death of pushrods and many other technologies over the years. And some technologies have made a revival due to a change in the requirements…like turbo’s, electric motors (Porsche were messing around with electric hybrid cars 100 years ago) and even Diesel engines themselves.

    No engineer set out with the mindset of “I’m going to design a pushrod engine”. They set out with an aim to design an engine that is small, light, can be manufactured for £x per unit, can achieve x bhp and y NM of torque, achieve z l/km economy, have a service life of x years and cost x per year to maintain and will meet all the noise and emissions regulations…..the technologies to achieve all of that are then selected. How are pushrods contributing to all those requirements vs OHV or any other technology…the first question might be is an internal combustion engine the right powerplant in the first place?

    whatgoesup
    Full Member

    Still no answer as to why push rod engines intrinsically have better bottom end power @trail_rat

    acknowledging that trail rat meant “good” not “better” and the others have already responded to this. My view is that…

    Pushrod engines do not have better low end power.
    2 valve heads generally have better low end power than 4v (generalisation). This is due to gas flow dynamics in the intake port, valve and heads.

    mrmonkfinger
    Free Member

    @markwsf

    sanity to the thread

    thank you

    so, do I have this right –

    at any design target RPM for any engine type, the ideal valve movement is always “as fast as possible” and as pushrods are not faster than OHC, pushrods mean always suboptimal efficiency/power

    then, all that, traded off against simplicity / ease of service / packaging (e.g. that new BMW boxer) / marketing (harleys, GM v8)

    whatgoesup
    Full Member

    +1 for @wobbliscott ‘S answer.

    Most engines are developments and derivatives of previous designs, but when there’s a large scale new design or clean sheet design the most experienced group of engineers will spend time determining the overall architecture. That’s how things like the newer smaller 3 cycl 1 litre turbo engines have come into being – technology has advanced enough that when a clean sheet design is made these are now feasible.

    the first question might be is an internal combustion engine the right powerplant in the first place?

    Absolutely correct – there are now feasible options to IC engines and rapid progress is happening, hence all of the various hybrid and electric options becoming available.

    whatgoesup
    Full Member

    @mrmonkfinger – thanks, I hope these replied are interesting / useful and not coming across as a bit lecture / I know better ish.

    at any design target RPM for any engine type, the ideal valve movement is always “as fast as possible”

    Correct. This is of course a generalization but the optimal opening / closing speed is generally faster than valve & cam technology allows for.

    and as pushrods are not faster than OHC, pushrods mean always suboptimal efficiency/power

    Pretty much. Pushrods generate higher forces for a given valve motion and speed, hence the achievable speed limit would be lower for the same valve design and speed. It is not acceptable to have engines overly low RPM limits, and also not acceptable to have engines which fail or have short lives hence the part of the design that gets compromised is the valve motion.
    You can also throw a lot of money at making the valvetrain lighter – carbon fibre / titanium pushdrods, exotic valves etc which helps but adds $$$

    then, all that, traded off against simplicity / ease of service / packaging (e.g. that new BMW boxer) / marketing (harleys, GM v8)

    Correct, it’s a trade-off as is all good engineering design.

    Re tradeoffs Harleys for example have a massively compromised design to allow for marketing. Their twins fire at 90 degrees offset, not 180 to give that Harley sound and lumpiness. But that fault feature is WHY people buy harleys.

    mrmonkfinger
    Free Member

    STW is not Michael Gove, we like experts 🙂

    thols2
    Free Member

    Pushrod engines do not have better low end power.
    2 valve heads generally have better low end power than 4v (generalisation). This is due to gas flow dynamics in the intake port, valve and heads.

    Yes. In principle, you could take a pushrod engine and build an OHC cylinder head with exactly the same arrangement of ports, combustion chamber, cam profile, etc., the only difference being how the valve actuation was done. In that case, you would expect pretty much the same power curve, the only difference would be the friction in the valvetrain. A thought experiment like that shows that there is nothing inherent in pushrods that give better low down torque.

    Toyota and Honda looked at turbocharging versus multi-valve OHC back in the 80s. They apparently came to the conclusion that DOHC was optimal for road cars. That would have considered cost, reliability, fuel economy, emissions, etc. Toyota started out with wide-angle high-performance cylinder heads, but they were expensive to produce so they used a cheaper narrow-angle design for standard cars. The high-performance versions were designed to rev hard and produce high power at high revs. The standard engines were designed to produce good drivability and economy. The torque curve of the engine isn’t directly due to whether it’s OHC or not, you could make a DOHC engine with massive low-down torque and not a lot of top-end power if you wanted to. A lot of those compromises are reduced now with variable length intake manifolds and variable valve timing, so you get a much wider usable power band.

    all that, traded off against simplicity / ease of service / packaging (e.g. that new BMW boxer) / marketing (harleys, GM v8)

    GM had a quad-cam V8 but dropped it in favour of a clean-sheet pushrod engine. That had nothing to do with marketing. They did their sums and decided that pushrods were a better solution for that particular situation. The current Chevy V8s use variable intake manifolds, variable valve timing, and direct injection. They rev to a bit over 6000 RPM and crank out over 400 HP. That’s about double the power output of what they were getting out of the old V8 designs in the 90s. It obviously wasn’t the pushrods that limited the old engines, it was the design of the intake and exhaust manifolds, porting, combustion chambers, and injection systems. Once those were modernized, the engines suddenly started putting out decent horsepower.

    whatgoesup
    Full Member

    Yes. In principle, you could take a pushrod engine and build an OHC cylinder head with exactly the same arrangement of ports, combustion chamber, cam profile, etc., the only difference being how the valve actuation was done. In that case, you would expect pretty much the same power curve, the only difference would be the friction in the valvetrain. A thought experiment like that shows that there is nothing inherent in pushrods that give better low down torque.

    I agree with all of this, except to add that in addition to friction in the valvetrain the mass would rise, hence inertia and highly likely that after extended running you’d be more prone to getting premature wear and failures due to higher forces (cam surface wear and cracking etc). Also likely no-follow conditions between the pushrod / follower and cam lobe. Ignore these and the short-term performance would be pretty much identical.

    To avoid these circumstances you’d have to back off and “soften” the cam profiles, the result would be a reduction in performance, efficiency and an increase in emissions.

    It obviously wasn’t the pushrods that limited the old engines, it was the design of the intake and exhaust manifolds, porting, combustion chambers, and injection systems.

    I agree. The pushrod system is just one of many factors in a design. They are perhaps unfairly seen as very limited as they tend to be associated with old designs where the overall engine performance is poor as a result of all of these different factors and the poor pushrod gets associated with all of that. Design a modern engine around a pushrod system and you both get the benefit of all the modern technologies but also get to optimize the design for the pushrods limitations, so you can take advantage of it’s plus points (package size and simplicity of drive for a V8) whilst designing around its limitations (valvetrain inertia).

    Same with the pushrods valvetrains we use in our industrial engines. They’re overall very high tech, and we design around the limitations, just like we do every other limit that exists in an engine (the there are hundreds if not thousands of limits and compromises involved in an engine design).

    mrmonkfinger
    Free Member

    That had nothing to do with marketing

    I dunno. It might have had something to do with marketing – smaller engine, mounted lower & further back, used in sports car, etc etc. I did google that the motor was about $400 less to produce using pushrods vs OHC & belts. That aside, some of those old US V8 engines had woeful gas flow, as you say, and probably not that difficult to improve on.

    mrmonkfinger
    Free Member

    BTW do we need to start another thread, ‘why are there no sleeve valve engines’ ?

    ampthill
    Full Member

    I teach A level physics for a living

    On one of our worksheets students estimate the acceleration of a piston in an engine. The bright students students always get embarrassed and ask where they’ve gone wrong when they get 20,000 m/s^2. I think the stroke length is probably a bit long but it makes a useful point.

    Using a stick to poke a valve open once is easy. Doing it 2000 times a minute, each time as quickly as possible, and the sticks inertia really matters.

    Great thread I’ve learnt loads

    thols2
    Free Member

    To avoid these circumstances you’d have to back off and “soften” the cam profiles, the result would be a reduction in performance, efficiency and an increase in emissions.

    No, the thought experiment was to take an existing pushrod engine and convert it to OHC with identical ports, chamber, and cam. The cam profile is already suitable for a pushrod engine.

    Thing is that older road car engines had pretty mild cam profiles, regardless of whether they were pushrod or OHC. Manufacturers weren’t putting the lumpiest possible cam in anyway, hence pretty much any stock engine could be improved just by getting the cam reground, rejetting the carburettor or remapping the injection, and tidying up the intake and exhaust. Often, the limitation on camshaft lift was to prevent the pistons striking the valves if the timing chain broke. It was nothing to do with the mechanical limitations of pushrods or OCH valvetrains.

    rsl1
    Free Member

    I think the thing that the people trying to pick holes in markwsf’s answers are not getting is the fact that, even if push rods could feasibly do the job, that alone is not good enough. For the automotive market, manufacturers are pushing to the very limit of what is possible with existing technology in order to meet emissions targets. Taking a step back to push rods simply isn’t possible whilst maintaining those targets, for the reasons markwsf described. Soon manufacturers will have to design to meet euro 7 and I suspect a lot of them are quite worried how they’re going to achieve that even without stepping backwards in technology.

    The LS series engines are a corner case that simply isn’t relevant to the majority of UK sales. Most cars sold here come under a portfolio that has to collectively meet fleet emissions regulations and you can’t have a sub-optimal solution needlessly dragging that figure down.

    thols2
    Free Member

    On one of our worksheets students estimate the acceleration of a piston in an engine.

    Does it take account of the conrod length? The shorter the conrod relative to the stroke, the higher the piston acceleration. A shorter rod will improve low-end torque, but cause higher side loading on the pistons. A longer rod will need a higher deck height, which will make the engine heavier and taller (or wider in the case of vee engines).

    https://www.enginebuildermag.com/2016/08/understanding-rod-ratios/

    whatgoesup
    Full Member

    Using a stick to poke a valve open once is easy. Doing it 2000 times a minute, each time as quickly as possible, and the sticks inertia really matters.

    Yes. Look up “high speed valve spring” or similar on youtube for some cool vids!
    The bit that still amazes me is that not only do the valves open and close that fast, but there is a carefully designed closing ramp so they don’t get “slammed shut”, but slowed down so the actual velocity at closure is quite low – to avoid multiple issues but mainly valve recession (wear of the valve/seat interface). We use software these days that takes into account the inertia of all the components etc but it just comes down to basic differentiation and integration of curves.

    the thought experiment was to take an existing pushrod engine and convert it to OHC with identical ports, chamber, and cam. The cam profile is already suitable for a pushrod engine.

    Oh, I see. Yes, this way around would work fine and the performance would be pretty much identical bar a slight frictional difference which would be lost in the noise anyway.

    Often, the limitation on camshaft lift was to prevent the pistons striking the valves if the timing chain broke. It was nothing to do with the mechanical limitations of pushrods or OCH valvetrains.

    Agree. The tools that are used to design and simulate these systems didn’t exist back in the day so there was a lot more trial and error which lead to more conservative designs. They’d push harder until issues were apparent and then try and engineer the issue out – which leads to improved materials, designs, calculation and analysis etc added to more experience hence progress. Still the same iterative process today just with more computational power.

    Interestingly, when diving into these topics WW2 aircraft engines are very often a major topic as the sudden need for development of increased power and longevity drove a vast range of improvements. For example the valve seat materials used on high performance engines (Stellite and other cobalt based alloys) came into use (in engines) in this period and are still one of the premium choices now.

    whatgoesup
    Full Member

    @rsl1 – thank you, you’ve summarized it much better than me.

    Even if the use of pushrods for mainstream automotive applications is/was “possible” it’s not the optimal solution so would not get used. As you say every design choice is very stringently chosen to be the best available and 99+% of the time OHC represents the best choice – it’s such a clear choice that its not a decision that takes long to make. Which is important as that decision making effort needs to go into other areas that do offer genuine choice.

    For example – what flavor of variable valve timing / lift is optimal etc. That’s a complex subject and there is not “right” answer – each company has it’s own preference & often proprietary systems and even they don’t apply the same system each time.

    Now add hybridization onto it and even the use of the Otto or Diesel cycles themselves gets challenged, with a lot of hybrids opting to use the he Atkinson cycle engine instead (for turbocharged industrial engines we often use the Miller cycle, which is closely related).

    molgrips
    Full Member

    Why are we still arguing about who has a better rock that their dads gave them to smash a nut, when there are nut crackers.

    If you’re talking about the shift to EVs then yes, it’s remarkable how almost this entire debate will be completely irrelevant in a decade or less, and the only people who know anything about it will be specialists designing for the remaining niche applications for ICEs. Just like all the stress over the condition of the various transmission and engine components in my Merc (see thread) or the black smoking issue on someone’s Nissan (see other thread). It’ll all go away.

    Re pushrods specifically in diesels – isn’t the instant force at detonation significantly higher in a diesel? Wouldn’t that result on greater shock loading on the pushrod, cams and rockers and hence require more heavily built parts and exacerbate the problem more than on a petrol engine?

    thisisnotaspoon
    Full Member

    Although a diesel doesn’t rev to the same high rpm as a petrol it’s still fast enough for the valvetrain dynamics to be a limiting factor

    Except that is the bit which is demonstrably false.

    The final(ish, it went on a lot longer in various guises) iteration of the BMC A-series in the Mini Copper S didn’t hit it’s peak until 5900rpm. And that was with the big (thus heavier) 2-valve head. 4 valves and 16 pushrods would each weigh less (or you could go down a development path of wider cams and elliptical followers to spread the load, etc)

    And the question was never phrased “why are there no new….” or “could you build a euro7…..”. It was why did they not persist a lot longer than they did. And I’m just not convinced cam profiles and rev limits explain it.

    Now add hybridization onto it and even the use of the Otto or Diesel cycles themselves gets challenged, with a lot of hybrids opting to use the he Atkinson cycle engine instead (for turbocharged industrial engines we often use the Miller cycle, which is closely related).

    Mazda use miller cycle engines in their non hybrids. Hence why They’ve gone down a path of really low specific output compared to everyone else.

    whatgoesup
    Full Member

    @molgrips

    isn’t the instant force at detonation significantly higher in a diesel?

    Happily not as during combustion the valve is closed and so the combustion forces are reacted through the valve seat and cyl head. If the valves are open during combustion you’ve got a whole different problem !


    @thisisnotaspoon

    Although a diesel doesn’t rev to the same high rpm as a petrol it’s still fast enough for the valvetrain dynamics to be a limiting factor

    Except that is the bit which is demonstrably false.

    It really isn’t – see below.

    The final(ish, it went on a lot longer in various guises) iteration of the BMC A-series in the Mini Copper S didn’t hit it’s peak until 5900rpm. And that was with the big (thus heavier) 2-valve head.

    Yes, you can design a pushrod engine to achieve high RPM. The use of pushrods at high RPM will just impose significant compromises on things such as the achievable valve speeds which matters massively for emissions and efficiency.

    If you’re happy with BMC A series levels of performance then sure, why not.

    And I’m just not convinced cam profiles and rev limits explain it.

    Bearing in mind that this field is what I do for a living, and if after everything I’ve written in this thread you still think that then I’m not going to bother trying to explain any more as you’re either not reading, taking the time to think this through, not believing me or are trolling.

    tillydog
    Free Member

    there is a carefully designed closing ramp so they don’t get “slammed shut”, but slowed down so the actual velocity at closure is quite low

    IMHO, this is one of the key limitations inherent in pushrod operated valves – the cam can only control valve closing while all parts of the valve train remain in contact with each other. If you increase the speed of the engine there comes a point where the inertial loads in the valve train cause some of its components to lose contact on the cam closing side. The valve is then free to accelerate until it slams into its seat causing rapid wear / mechanical failure. It can also happen that the floating valve doesn’t close in time to avoid the rising piston – especially important for diesel engines with their high compression ratio / small piston to valve clearance.

    To echo Markwsf – there’s no intrinsic characteristic of pushrod vs OHC that affects power delivery, other than pushrod valvetrains inherently have more inertia than OHC type valvetrains which either limits RPM for a given valve lift curve, or enforces a more conservative valve lift curve to achieve a given RPM.

    The power delivery of an engine can be tuned to whatever is required within these restrictions.

    Other reasons for the demise of pushrod engines: “manufacturability” – OHC designs generally have lower parts count and are more suited to automated assembly; Efficiency – lower valve train inertia for OHC can be exploited to offer better power to weight for a given reliability or reduced fuel consumption for a given power output; Noise – there are fewer mechanical interfaces in an OHC design, so fewer noise sources, and they all tend to be concentrated in one place.

    Now then, why aren’t there more side valve engines? 😀

    whatgoesup
    Full Member

    @molgrips – apologies I didn’t quote the full piece of your post that I meant to and too late to edit – please disregard and I’ll re-post below

    isn’t the instant force at detonation significantly higher in a diesel? Wouldn’t that result on greater shock loading on the pushrod, cams and rockers

    Yes, the actual instantaneous pressure is (generally) higher – plus Diesels run higher CR’s so the pressures are higher anyway. Happily this force is not transmitted through the valvetrain as during combustion the valve is closed and so the combustion forces are reacted through the valve seat and cyl head. If the valves are open during combustion you’ve got a whole different problem !

    The whole structure of a Diesel does generally need to be able to survive higher pressures though – so blocks, heads, valve heads, pistons, con-rods, cranks etc are all usually larger & heavier. Clearly there are exceptions to this for various reasons but as a general trend it’s true.

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