• This topic has 176 replies, 32 voices, and was last updated 1 year ago by thols2.
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  • Why are there no diesel pushrod engines?
  • thisisnotaspoon
    Full Member

    I figured out the other day that the cambelt in my car will likely outlive the car. It was replaced at 100k so the 2nd one will likely be 20 years old and 200k and I doubt the rest of the car will last to 2025).

    But then I couldn’t figure out why it had a cambelt (or chain) in the first place?

    So I googled it, and all that comes up is a lot of American sites explaining why engines are still built with an OHV configuration still have pushrods rather than an OHC.

    They obviously have limitations on small revvy petrol engines, but why did they disappear from diesels?

    trail_rat
    Free Member

    Double overhead valves mainly.

    Push rods give good low end but lack at high end /lack revs also

    See also block real estate

    Valve train mass

    And noise/smoothness of engine.

    Pushrods in the modern world would be like turning up to the tdf with a Viking 9 speed

    bigyan
    Free Member

    You get diesel pushrod engines, just not common in the UK.

    trail_rat
    Free Member

    mostly In torquey applications rather than civilian

    RustyNissanPrairie
    Full Member

    Landrover’s 300tdi was the last diesel pushrod engine?

    I imagine that line boring of the camshaft in the push rod block and locating the bearings is an added production line hassle.

    wobbliscott
    Free Member

    Struggling to think of the benefit for general purposes. Low down torque is not lacking in most diesel cars. Pushrods increase parts count, complexity, probably limit the modular design of the engine so increase costs when it comes to using the same core engine design but scaled for different capacities and cylinder counts, might cause issues with turbocharged applications, are not going to do anything to improve emissions or noise…the two principle design criteria of a modern car engine. Probably these and many other issues. All these tings have a cost vs benefit analysis done and push rod engines obviously lose out in an age where we’re chasing ever more efficiency from engines.

    Not sure how a pushrod is an improvement over a cam acting directly onto a hydraulic tappet then the valve.

    duncancallum
    Full Member

    Theres load of older engines

    Nissan 2.7 that’s in black cabs for starters

    jmmtb
    Full Member

    Land Rover 300 Tdi was an overhead cam, not a pushrod engine. Ironically (IIRC), the 300 Tdi was an update on the 200 Tdi requested by the military to allow easier access to the cam belt change.

    trail_rat
    Free Member

    Cannot confirm if it has an overhead cam as I don’t have one

    But the 300tdi absolutely has pushrods.

    Same ones my 200tdi has.

    PN 546799

    No one really wants a car that sounds like a tractor again do they ?

    singletrackmind
    Full Member

    Never going to happen, unless someone shed builds one, but i would love to see a deisel elec hybrid
    Tiny 1ltr tdi engine, vw did a 3cyl version, plus tiny say 30kw elec pack
    Plug in overnight to charge then alternate between elec or derv, with regen or trickle charge.
    The range could be huge, great for mixed use driving say through a major city then on to motorway or open roads

    julians
    Free Member

    Pushrods in the modern world would be like turning up to the tdf with a Viking 9 speed

    I give you the chevrolet Ls series small block v8 – with the LT2 variant still used in the most recent corvette C8.

    https://ls1tech.com/articles/2020-corvette-c8-why-no-dual-overhead-cam-engine-gm/

    trail_rat
    Free Member

    I give you the chevrolet Ls series small block v8 – with the LT2 variant still used in the most recent corvette C8.

    I am aware of that. It’s also the butt of many jokes about lack of innovation within that that particular company. It’s also not a diesel.

    a11y
    Full Member

    Pushrods increase parts count, complexity,

    I was under the impression pushrods resulted in simpler engines with less complexity? And also more compact overall engine size and generally weigh less too – I’ve got an LS2 and it’s remarkably small within the engine bay given the capacity.

    I really don’t know why they’ve not been an prevalent in diesels though, given the low-down torque general characteristics of pushrod designs.

    a11y
    Full Member

    I am aware of that. It’s also the butt of many jokes about lack of innovation within that that particular company. It’s also not a diesel.

    And I’m clearly a little biased given my own, but much I’d rather have something like an LT2 over a modern turbo’d 4 or 6-cyl with all the complexity. I’m hanging onto my old school engine as long as I can.

    trail_rat
    Free Member

    Im of similar opinion but that doesn’t make it good in the modern world for a variety of reasons.

    Certainly wouldn’t warrent designing a slew of new pushrod engines.

    thols2
    Free Member

    General Motors in the U.S. still uses pushrod 2V OHV in their V8 petrol and diesel engines. American style V8s generally have large cylinder bores and shorter strokes, so you can still get a large valve area with 2 valves. The cylinder heads are simpler to cast and the parts count is lower. You only need 1 camshaft instead of 4, 16 valves instead of 32, etc. A single cam with 16 valves has lower friction than four cams with 32 valves, so that helps fuel economy. DOHC 4V isn’t automatically better for every application.

    trail_rat
    Free Member

    so that helps fuel economy

    Tenuous link when for most of the other reasons you’ve outlined why they work for American v8s that a given massive displacement the bhp and the fuel economy is frighteningly low. That’s another reason we have moved on from it here in the UK.

    molgrips
    Full Member

    And I’m clearly a little biased given my own, but much I’d rather have something like an LT2 over a modern turbo’d 4 or 6-cyl with all the complexity. I’m hanging onto my old school engine as long as I can.

    What Trail Rat said. You might be nostalgic for mechanical simplicity but what you call simple I call crude.

    julians
    Free Member

    What Trail Rat said. You might be nostalgic for mechanical simplicity but what you call simple I call crude.

    Its all moot anyway – given the rise of the brushless electric motor and the level of simplicity that brings compared to any flavour of internal combustion engine.

    I’m hoping to make excellent use of all the ‘crude’ sportcars that nobody else will want because they’ve been made obselete by electric motors over the next 10 years*

    * probably wont happen , as all the other sportcar fans will have the same idea.

    K
    Full Member

    Ohv is only a real benefit in something like a larger V8 engine where ohc is going to make the engine taller and wider with more mass further away from the crank. Less cam drive train as you only need one cam at the bottom of the valley thus making it cheaper to produce. Or ease of maintenance on parts that could actually be redundant. Or cheap because the tooling is already paid for 30 plus years ago…

    OHV engines tend to be large capacity engines so producing torque isn’t really being compromised by what drives the valves.

    Hey why not just use big diesel 2 strokes like they do in the marine vessels…

    Isn’t any other justification is a bit like saying my bikes suspension linkage is the best because it’s on the bike I own and that’s what I’m used to. Why don’t we argue for the return of URT suspension as it’s simplicity is far superior?

    A guy that came to my workshop a couple of years ago wanting help to finish making a “revolutionary” supercharged, V twin, side valve, pushrod engine prototype that was in his eyes going to change the world. I had to try and tell him it wasn’t something I would be comfortable to spend his money and put the time in to. That in the past 70 years there have been some fair advances in technology. And that it might be worth looking at the efficiency, power and service schedules that was available from the current generations of production engines that have had many  thousands of hours of development over several generations.

    toby
    Full Member

    Possibly also a factor that until relatively recently a diesel would have needed a long cambelt to drive a mechanical injector pump, so the advantages of keeping a short timing chain wouldn’t apply. More recently people want quieter, higher revving, less “dieselly” engines.

    thols2
    Free Member

    so that helps fuel economy

    Tenuous link when for most of the other reasons you’ve outlined why they work for American v8s that a given massive displacement the bhp and the fuel economy is frighteningly low. That’s another reason we have moved on from it here in the UK.

    If you look at the Australian touring cars (i.e. V8 Supercars), they run Nascar based pushrod V8 engines for the GM and Ford teams. Nissan, Volvo, and Mercedes were also involved but withdrew, they used DOHC V8s. Because of the constraints in the regulations (7500 RPM limit, plus restrictions on valve lift and duration), the DOHC engines weren’t any more powerful but the Nascar engines had better fuel economy.

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

    The assumption that OHC is inherently better and pushrods are “crude” is too simplistic. It depends on the application, for some applications, OHC is an unnecessary complication.

    mcmoonter
    Free Member

    300tdi engines are definitely pushrod engines. I’ve had the head off of mine

    squirrelking
    Free Member

    In the marine world only 4 stroke generators are running on pushrods, the large 2 strokes aren’t even running camshafts any more and instead rely entirely on electronic timing and actuation. That’s where the real savings are.

    thisisnotaspoon
    Full Member

    Double overhead valves mainly.

    Push rods give good low end but lack at high end /lack revs also

    That’s just a function of the cam you put in it though? Before the 90’s there were plenty of rev happy pushrod petrol engines. And it fairly moot on a turbo diesel too because they inherently don’t rev.

    Double overhead valves mainly.

    Pushrods disappeared first though? And 4-valve pushrods been done.

    But bringing it back specifically to turbo diesels, forced induction narrows the gap between 2-valve and 4-valve. On a NA engine you’re trying to minimize pressure drops so every fraction of a psi gained by polishing the intake, higher lift cams, more valves etc is of benefit. A turbo is just throwing 15psi against it. It’s why any old spotty teenager with a laptop can set themselves up as a diesel tuner.

    Struggling to think of the benefit for general purposes.

    It wasn’t so much a question of benefit, more of a thought about why we changed?

    Pushrods increase parts count, complexity,

    No, it’s definitely simpler. Especially to assemble and work on. Taking the head off a pushrod engine takes minutes.

    Not sure how a pushrod is an improvement over a cam acting directly onto a hydraulic tappet then the valve.

    Not saying it’s better, but for a diesel where the power runs out at 3000 and the redlines 5000rpm, I’m not really seeing why we didn’t go down the route of hydraulic tappets on pushrods.

    If nothing else you lob 150mm off the height of the engine (aerodynamics, pedestrian safety etc).

    The best argument I could think of was where you have petrol and diesel engines built from the same block so it’s saving tooling.

    molgrips
    Full Member

    for a diesel where the power runs out at 3000

    That hasn’t been the case for a long time.

    trail_rat
    Free Member

    If nothing else you lob 150mm off the height of the engine (aerodynamics, pedestrian safety etc).

    Tell that to my pushrod diesel.

    Tallest engine I’ve ever seen lb for lb

    trail_rat
    Free Member

    for a diesel where the power runs out at 3000

    That hasn’t been the case for a long time.

    Don’t you have the 1.6 16v diesel in your Berlingo Tina’s

    tthew
    Full Member

    but i would love to see a deisel elec hybrid


    @singletrackmind
    , I’m pretty sure this is how the hybrid Ford Transit Custom works. It’s bloody expensive too!!

    molgrips
    Full Member

    Peugot had a diesel hybrid too. 3008 – not sure if there’ve been any more.

    whatgoesup
    Full Member

    I’ve worked in Diesel engine design and development for the last 10+ years, including automotive and industrial size engines (1.6 – 78 litres) – currently working at the industrial end of things.

    In short, side-cam / push rod engines have many disadvantages and only a few advantages, which means that they’re only used on larger, industrial engines.

    Disadvantages:
    High mass of the whole pushrod/rocker system leading to valve dynamics issues, i.e. max rpm limitations. This leads to compromises on the cam profiles that can be used and also the weight of the valve becomes important, hence the design has to be compromised to keep weight down.

    Higher cost & part count – all of those additional parts – the pushrods and larger, more complex rockers add parts cost, weight and assembly time (important for high volume manufacture)

    Advantages:
    The main one – cylinder heads can be removed without removing the camshaft. Industrial engines usually have individual cyl heads and can run through several sets of cylinder heads per every major engine rebuild so this is important. Car engines typically have one head for all cylinders on a bank and also have fit-for-life cyl heads so little need to be able to remove the head during normal service life. Note industrial engine can have a time to rebuild in the 20-80K hour range.

    More durable gear rather than chain or belt drive for the camshafts is more straightforward. Important for life reasons. Also loadings are a lot high on industrial engines, especially with high cylinder counts – for example a 60L V16 has a lot of cam load.

    TL:DR – For automotive engines pushrods are more expensive and impose limitations. Overhead cams are cheaper and have less limitations.

    thisisnotaspoon
    Full Member

    Don’t you have the 1.6 16v diesel in your Berlingo Tina’s

    2.0, so it’s a SOHC and no intercooler.

    Hence my thoughts on why if they clearly weren’t fussed about intake density or pressure drops, why did it need an OHC.

    whatgoesup
    Full Member

    why did it need an OHC. why did it need an OHC.

    As per my previous post – because it’s cheaper and both better performance plus easier to engineer a decent performance.

    Why would they have designed a more expensive, lower performance, more expensive to develop engine ?

    trail_rat
    Free Member

    2.0, so it’s a SOHC and no intercooler.

    Same block used in the 16v version in the b and c varients fitted to most things except the Berlingo is the likely answer for the tractor engine not being push rod.

    thols2
    Free Member

    TL:DR – For automotive engines pushrods are more expensive and impose limitations. Overhead cams are cheaper and have less limitations.

    GM looked at all this with their V8s, but stuck with pushrods. They obviously decided that the cost/performance benefits for that application (large capacity V8) made pushrods competitive.

    Variable valve timing would be cheaper on a single camshaft engine compared to DOHC on a vee, surely.

    whatgoesup
    Full Member

    GM looked at all this with their V8s, but stuck with pushrods.

    I think that’s likely a combination of marketing (Good old fashioned all American V”) and the cost of redesigning and retooling their base engine block.

    Variable valve timing is usually done via either manipulating the belt runs, or via a variable cam pulley (the inner and outer can move relative to each other) – in the latter case you can manipulate the intake and exhaust timings independently on a DOHC engine, whereas with SOHC cam you only move them together, but of course is cheaper (one fancy cam pulley needed not two).

    wobbliscott
    Free Member

    GM looked at all this with their V8s, but stuck with pushrods.

    I think that’s likely a combination of marketing (Good old fashioned all American V”) and the cost of redesigning and retooling their base engine block.

    This and a case of if they’re selling like hot cakes then why change? Making product changes is horrendously expensive and very disruptive so you only change your product in response to competition or changes in regulation. Other factors also but mainly those two. With a whole industry of aftermarket parts out there it makes the engine very accessible for the DIY mechanic/hot rodder who might be their main customer base.

    thols2
    Free Member

    I think that’s likely a combination of marketing (Good old fashioned all American V”) and the cost of redesigning and retooling their base engine block.

    They completely redesigned the Chevy small block back in the 90s, then again about 10 years ago. The new engines have the same general layout and bore spacing as the classic small block, but nothing else was carried over. They also had a different line of V8s used by Cadillac, with DOHC layout (plus the old DOHC V8 they used in the Lotus Corvette back in the 90s). They dropped the DOHC Cadillac engine in favour of the pushrod Chevy engine. The pushrod engine has variable valve timing and other modern technology, so the GM engineers clearly decided that a single cam OHV layout had advantages for a big V8. The performance versions are cranking out 450 to 500 BHP in stock spec, so the “crude” technology doesn’t seem to be too big of a handicap.

    Nascar engines crank out 800 HP and rev to 8500 to 9000 RPM, so pushrod 2V engines aren’t as terrible as people imagine.

    trail_rat
    Free Member

    The performance versions are cranking out 450 to 500 BHP in stock spec, so the “crude” technology doesn’t seem to be too big of a handicap.

    Nascar engines crank out 800 HP and rev to 8500 to 9000 RPM, so pushrod 2V engines aren’t as terrible as people imagine.

    Both attributes 95% of the car driving population need in spades. How are the economy and emissions compared to a non 2v non V8 option.

    julians
    Free Member

    Both attributes 95% of the car driving population need in spades. How are the economy and emissions compared to a non 2v V8 option.

    Comparable/better than other (less ‘crude’ ) engines of similar power/usage/category it would appear

    https://www.motortrend.com/news/c8-corvette-official-fuel-economy-mpg/

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