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

    This is a good thread. 🙂

    molgrips
    Full Member

    If the valves are open during combustion you’ve got a whole different problem !

    On that cylinder, yes – but what about the others? In a 4cyl isn’t one exhaust valve opening whilst combustion is happening on another?

    I do find engine design fascinating but I can’t wait for the time when they aren’t on our roads.

    whatgoesup
    Full Member

    On that cylinder, yes – but what about the others? In a 4cyl isn’t one exhaust valve opening whilst combustion is happening on another?

    The valves should only be opening and closing when there is fairly low pressure in the cylinder.

    In the “ideal” Diesel cycle as per the PV diagram below that’s exactly at bottom dead centre, and all of the gases instantaneously disappear out of the cylinder.

    The reality is that there is a finite time for the valve to open, and a finite amount of time for the gases to leave the cylinder, during which time the piston is still moving (one of the reasons for the “open and close valves as fast as possible” goal).

    So there will be some pressure against which the valve has to open, but nowhere near full combustion pressure.

    For comparison a “real” PV diagram looks more like this

    One of the goals in IC engine design is to get the real-world PV diagram as close to the ideal one as possible, as all of those “missing corners” represent energy that’s being lost.

    I do find engine design fascinating but I can’t wait for the time when they aren’t on our roads.

    I fully agree. In the long term the engineering skills are all transferrable, and in the short-medium term there is actually a lot MORE engine development going on as the demands on engines are changing rapidly, from being the only source of power to being part of a more complex system.

    One simple example is turbo lag. It used to be a really big deal, and compromises were made to minimise it. With advent of hybrids an electric motor can pick up the load instantly, so turbo lag is much less of an issue, hence the system can be better optimised for efficiency, power and emissions.

    wobbliscott
    Free Member

    I do find engine design fascinating but I can’t wait for the time when they aren’t on our roads.

    Even if they’re running on Hydrogen? If you mean you cant wait for a time until there are no cars on the roads full stop, then it will be a cold day in hell till that happens. But if its from an oil burning environmental point of view then Hydrogen is well and truly in the future of cars…either in the form of Hydrogen fuel cells or hydrogen burning ICE’s. BEV’s are not going to do it for everyone.

    thols2
    Free Member

    In a 4cyl isn’t one exhaust valve opening whilst combustion is happening on another?

    A 4 cylinder crankshaft looks like this.

    Cylinder 1 and 4 are 360 degrees apart (a complete 4-stroke cycle is 720 degrees, with 1 stroke taking 180 degrees of crankshaft revolution). Cylinders 3 and 2 are 180 and 540 degrees behind cylinder 1. When cylinder 1 is on its firing stroke (both valves closed), cylinder 4 is on its intake stroke (intake valve open), cylinder 3 is on its exhaust stroke (exhaust valve open), and cylinder 2 is on its compression stroke (both valves closed). It’s not quite as simple as that because there is overlap between the intake and exhaust valves, so the intake valve starts to open before the exhaust valve is fully closed.

    whatgoesup
    Full Member

    Re Hydrogen.
    For all-day use type applications batteries won’t cut it in the near future, so Hydrogen is being actively developed, both as an alternative fuels to use in developments of existing engines and also for fuel cells.

    Of course, this relies on producing the hydrogen without burning fossil fuels in the first place, via windfarms, solar, tidal, nuclear or other means. Hydrogen isn’t really a “fuel” so much as an energy storage medium.

    There is currently large investment in just this. E.g. something from my company I can share as it’s public – a 500 with potential for 100MW electrolizer site in Spain.

    Cummins Press Release – Hydrogen Plant

    thisisnotaspoon
    Full Member

    trolling.

    I’ve not disagreed with anything technical you’ve said, that was other people,.

    I just think the argument that in most cases they shared castings with the far more popular petrol versions in the 90’s makes more sense.

    The performance argument doesn’t hold up because they don’t rev anywhere near a 70 year old petrol production engine design. And the cam profiles of turbo diesels aren’t anywhere near as aggressive as NA petrol engines, there’s neither the need nor the ability high compression means less valve overlap (need) and less clearance to the piston until further around TDC (ability).

    And that the emissions argument falls down on things like I don’t think it’s possible to build a DOHC unit injector cylinder head in a car engine sized package? The injectors are just too bulky. So for the pre DOHC engines the complexity argument doesn’t hold up as DOHC wasn’t an option.
    So Euro 4 wasn’t possible without common rail, but DOHC wasn’t possible without it , not the other way around . And SOHC persisted for at least another generation of emissions rules.

    whatgoesup
    Full Member

    I’m sorry @thisisnotaspoon but you are incorrect and I’m not going to debate this any more with you as you’ve ignored most of my explanations that I’ve spent quite some time and effort on.

    molgrips
    Full Member

    Hydrogen is well and truly in the future of cars

    Nah I totally disagree. We’ve already got the BEV cars, now we just need them to be cheaper, we need more charging stations, and there needs to be on-street charging. The first two are happening pretty quickly, and the on-street charging is already being worked on. The massive advantage, apart from mechanical simplicity, is the fact that electricity supply is already everywhere and we have plenty of it.

    The number of battery discoveries and innovations every month is pretty high, not least because half the modern economy benefits from it not just cars so many industries are investing in it. Right now, BEVs work, provided there is access to charging. Hydrogen vehicles really don’t, and we’d need huge technological developments and infrastructure investment to do that. So why bother, when BEVs work and are only going to get better?

    EDIT the above is all about passenger cars and vans btw.

    molgrips
    Full Member

    And that the emissions argument falls down on things like I don’t think it’s possible to build a DOHC unit injector cylinder head in a car engine sized package?

    Not sure what you mean – is this an example of an argument that’s null, or you think that’s currently the case?

    My Passat is a DOHC engine with unit injectors, and it’s Euro 4.

    whatgoesup
    Full Member

    Hydrogen is well and truly in the future of cars

    My personal view is that for passenger cars BEV’s will be dominant, and pretty quickly.
    We’re already up to 200-300 mile range which is plenty for the vast majority of journeys, and technology is progressing rapidly. Infrastructure will need to develop, but with future smart grid type solutions becoming a reality that will help.

    For uses where battery capacity will be more challenging, e.g. HGV’s, excavators, trains etc I see Hydrogen as being dominant for some time. Likely initially using it in IC engines, transitioning to fuel cells in the future.

    molgrips
    Full Member

    I wonder about powering an HGV on hydrogen. I am not sure you’d be able to get enough H2 in a sensible sized fuel tank to make much of a long journey.

    H2 has about 2.6 times the energy per kg of diesel, but even in liquid form it’s 11 times less dense, so that means a tank 4x as big as the current HGV tanks even if you were able to liquify it which itself takes a hell of a lot of energy. Diesel’s already liquid.

    And you’d not be able to liquify it in practical uses I don’t think.

    Trailseeker
    Free Member

    Toyota are doing a lot of development with H2 both ICE & fuel cell/electric & say they could have a truck ready for sale in two year.

    https://www.thinkgeoenergy.com/toyota-tapping-hydrogen-produced-with-geothermal-energy/

    https://apnews.com/article/technology-joe-biden-business-environment-and-nature-climate-change-5913fff50378b5706769463a2b02dfca

    whatgoesup
    Full Member

    Agree. Aside from the production itself efficient liquidation, transport and storage are big deals.

    A truck fuel tank isn’t that large, so a tank 4x larger would be a viable solution especially given it would likely be designed in. It would likely actually be more than 4x bigger as the tank will need to be both stronger and (I think?) insulated.

    Once batteries are capable of displacing hydrogen in trucks they will I think. It’ll just be simple economics defining which is used as technologies develop.

    whatgoesup
    Full Member

    We’ve got a train out there running already – 180k km in so far

    Cummins Hydrogen Train

    Train

    mogrim
    Full Member

    markwsf
    Free Member


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

    I’m certainly enjoying them. Will have to move on to YouTube to see some of films of pushrods in action soon, though 🙂

    thols2
    Free Member

    H2 has about 2.6 times the energy per kg of diesel, but even in liquid form it’s 11 times less dense

    It also burns at very high temperature, so that causes NOx pollution. To lower the combustion temperature, the engine has to run very lean, so you get much less power for the same capacity engine.

    I think fuel cell vehicles are much more efficient than hydrogen ICEs. Fuel cells might have a future, I doubt that hydrogen ICEs do except as a niche thing.

    molgrips
    Full Member

    Hmm, re HCVs one would assume it’s possible to use the same tech as a BEV and just stick a h2 tank instead of a battery. It’d be packaged differently of course but we’ve already shown it’s possible with things like the Hyundai Kona. You’d have the front end of the BEV version and the body and back end of the petrol version with a different fuel tank.

    Because drivers’ earnings depend on fares, Goldstone said, “if they have to spend 40, 50 minutes, an hour, two hours plugging a car in in in the middle of the working day, that for them is just not acceptable.”

    Good job it doesn’t take that long then.

    wobbliscott
    Free Member

    Nah I totally disagree. We’ve already got the BEV cars, now we just need them to be cheaper,

    And what about sustainable? How can anything that relies on digging rare crap out of the ground and putting it through complex, energy hungry and polluting processing before it can be used? Sounds familiar by any chance??

    Batteries are not sustainable and no amount of technology into re-processing will change that. It is not scalable for demand. Hydrogen is the most abundant element in the universe…75% of the universe is Hydrogen and we can create as much as we want easily and cleanly and the byproducts of its use are just water.

    The future will be a mix of all kinds of technologies…including conventional ICE engines burning sustainable synthetic fuels.

    H2 has about 2.6 times the energy per kg of diesel,

    Damn sight better energy density than batteries. Nothing can beat diesel and petrol…whatever the solution compromises will have to be made. The specific application will determine which compromises will be made. Domestic cars BEV might very well dominate…commercial applications then synthetic fuel burning ICE hybrid or hydrogen fuel cell might win out, big lorries and other industrial vehicles then hydrogen is by far the most sensible solution. It’ll be Horses for courses.

    I wouldn’t be at all surprised if diesel and petrol engines will ever be fully retired. There will be a few very niche uses where only petrol or diesel will do.

    whatgoesup
    Full Member

    also burns at very high temperature, so that causes NOx pollution. To lower the combustion temperature, the engine has to run very lean, so you get much less power for the same capacity engine.

    True.

    Modern natural gas engines (methane, similar to hydrogen) run very lean indeed, and this is linked directly to NOx. Airflow becomes important, hence high pressure ratio turbocharging, high energy ignition systems and significantly worse transient performance compared to Diesel. The resultant engine out emissions are very good though, so whilst diesels need aftertreatment lean burn NG often does not. That’d be a challenge if it was a direct replacement in a vehicle so would need to part of a hydbrid type system (likely IC engine driving a generator then the vehicle is electric drive which is quite a common arrangement already)

    thols2
    Free Member

    would need to part of a hydbrid type system (likely IC engine driving a generator then the vehicle is electric drive which is quite a common arrangement already)

    I’m pretty sure a fuel cell EV would be more efficient.

    molgrips
    Full Member

    And what about sustainable? How can anything that relies on digging rare crap out of the ground and putting it through complex, energy hungry and polluting processing before it can be used? Sounds familiar by any chance??

    It does sound familiar, and that’s the point. I’m not saying BEVs are necessarily the best choice, I’m saying that’s what we’ll get. And in the future, because BEVs will be well established, they’ll come up with new battery tech that won’t be as problematic. If you are a FB user find phys.org and like the page – you’ll get a few stories a week about developments that are solving these issues. Because now, there’ll be a shit-ton of money behind it. Companies are paying techies to tinker in labs, this is really really cheap. For widespread H2 adoption we’ll have to invest massively in infrastructure and simply hope that it’ll take off. That’s a harder sell to investors.

    Batteries are not sustainable and no amount of technology into re-processing will change that.

    Odd thing to say – there is clearly SOME amount of technological development that will change it – what’s not clear is if that’s possible or not. There are lots of ways to make batteries, I think sodium ion batteries are a front runner.

    Damn sight better energy density than batteries.

    But as we’ve said, BEVs work. If you view H2 and BEV as competing technologies, I can’t see that H2 will win out for a long time, given the political and economic climate even if the storage problems can be overcome.

    whatgoesup
    Full Member

    would need to part of a hydbrid type system (likely IC engine driving a generator then the vehicle is electric drive which is quite a common arrangement already)

    I’m pretty sure a fuel cell EV would be more efficient.

    I agree. If the IC engine approach is used I would only imagine this being an interim as fuel cells develop. I’m not up to speed on fuel cell capabilities or developments so don’t know how likely this would be.

    The later part of this thread reminds me it’s time to start getting a bit more knowledgeable around the new techs to avoid getting stuck in the past !

    molgrips
    Full Member

    I wonder how they’d stop water condensing everywhere on a cold start with an H2 ICE engine? I mean they have made it work so presumably they’ve decided it’s not an issue. Maybe you’d need a bleed valve on the exhaust like you have on brass instruments!

    wobbliscott
    Free Member

    But as we’ve said, BEVs work. If you view H2 and BEV as competing technologies, I can’t see that H2 will win out for a long time, given the political and economic climate even if the storage problems can be overcome.

    Well BEV’s don’t really work do they? I guess it depends on what you define as work. They move, but they’re nowhere near as convenient as a petrol or diesel engine interns of range and time to charge vs. filling up a tank of fuel.

    But H2 engines also work by the same comparison…its established technology both in fuel cell form and ICE form. Yes there is a challenge with distribution but you can generate hydrogen anywhere by renewable electricity generation so less of an issue even compared to oil and petrol where it has to be moved all over the country. Hydrogen can be generated locally. Yo. might not get as much range out of a tank of H2 but if it only takes a minute to fill up compared to 30 mins or so for a BEV then its not a huge problem.

    Odd thing to say – there is clearly SOME amount of technological development that will change it

    Well improve it but not transform it. The best Elon Musk has come up with for spent car batteries is to repurpose them in his power walls which is just kicking the can down the road…what then once the battery performance degrades beyond being useful for that? You’re into heavy, energy hungry, expensive and polluting reprocessing of precious earth elements.

    But anyway, my point was that these technologies and solutions in the future world wont be competing. There wont be a one single solution for everything. There will be a number of different solutions for different applications. We’re never ever going to see battery powered aircraft for example. Well airliners at least.

    whatgoesup
    Full Member

    There will be a number of different solutions for different applications.

    This.

    It will be dynamic and change over time as technologies and markets develop.
    There isn’t and doesn’t need to be a one-size fits all solution.

    There might even be the odd pushrod diesel engine in the mix 😉

    thols2
    Free Member

    I wonder how they’d stop water condensing everywhere on a cold start with an H2 ICE engine?

    It’s not a problem as far as the engine operation goes. The same thing happens with petrol or natural gas engines. Hydrocarbon fuels are compounds of hydrogen and carbon. When that is oxidized, the hydrogen combines with oxygen to form water, the carbon combines with oxygen to form CO2. If you only do a short run in a petrol car, the exhaust system doesn’t get hot and the water vapour in the exhaust condenses. Combined with CO2 and NOx, this forms carbonic acid and nitric acid, you basically get acid rain in the exhaust. If you only do very short runs in a car, the exhaust system will rot out because it never gets hot enough to evapourate the water.

    molgrips
    Full Member

    The same thing happens with petrol or natural gas engines.

    I’d have expected much more with a H engine.

    wobbliscott
    Free Member

    Well not sure of the chemistry of it, but hydrogen engines can run a hell of a lot leaner than petrol or diesel engines. Like upto ten times leaner, one of the many benefits of H2 over petrol and diesel, so you don’t need to add so much fuel to start when the engine is cold, so less fuel in the mix means less H2O production. Not sure wether H2 produces more or less H2O relative to petrol or diesel but can’t see how it is a problem or a blocker. There are more tricky problems to sort out to productionise H2 engines rather than concerns around water condensing in the exhaust.

    Of course using H2 as a fuel for aircraft means production of more contrails which have a global cooling effect due to reflecting solar energy back into space so a potential proactive way to combat climate change. Over the 24hrs following the global grounding of aircraft immediately after 9/11 there was a definite step increase in global temperatures measured as a result of an instant removal of contrails reflecting solar energy.

    thols2
    Free Member

    so less fuel in the mix means less H2O production.

    It also means less power. To make the same power, you need to burn the same amount of fuel, provided that’s done with the same thermal efficiency. Therefore you need a larger capacity engine to get the same performance out of a hydrogen ICE. You’re burning the same amount of hydrogen, so you’ll produce the same amount of water. The lean burn engine will produce much less NOx though because the combustion temperature is lower.

    I’d have expected much more with a H engine.

    A typical petrol molecule is C8H18 (octane). That will produce eight molecules of CO2 and 9 molecules of H2O. Hydrogen has an atomic mass of 1, carbon 12, and oxygen 16. If my arithmetic is correct, one mole of octane has a mass of 114 grams and will produce 372 grams of CO2 and 162 grams of water when burnt. As a ballpark figure, a litre of petrol will produce roughly a litre of water when burnt.

    A kg of hydrogen will produce 18 kg of water. So yes, a hydrogen vehicle will produce more water, but petrol vehicles also produce a surprising amount of water and it’s not a problem as long as the vehicle is run long enough to get the exhaust system hot.

    molgrips
    Full Member

    hydrogen engines can run a hell of a lot leaner than petrol or diesel engines

    How come? (serious question)

    thols2
    Free Member

    hydrogen engines can run a hell of a lot leaner than petrol or diesel engines

    How come? (serious question)

    That statement isn’t really correct. Petrol engines generally run close to a stoichiometric fuel-air mixture, which means that there is exactly enough oxygen to burn every bit of fuel. For maximum power under acceleration, it might be slightly rich, this extra fuel helps to cool the charge. For maximum economy while cruising, it might be slightly lean. With carburetors and port fuel injection, this was necessary to get the charge to burn properly. If the mixture is too lean, you’ll get combustion problems. Also, you need to keep the combustion temperature low enough to meet NOx emission standards. After emissions rules were introduced in the 1970s, compression ratios had to be reduced to lower the combustion temperatures, which caused huge drops in power output for performance cars. Direct injection engines can run leaner by injecting fuel so that there’s a rich mixture around the spark plug and a leaner mixture elsewhere. Compression ratios on modern direct injection systems seem to be much higher too, obviously they have put a lot of work into understanding the combustion process and how to control combustion temperature. Any modern engine has an oxygen sensor in the exhaust and continually adjusts the fuel-air mixture according to the conditions.

    Diesel engines use a completely different combustion process. They have a very high compression ratio and direct injection. The air charge is compressed so that it’s hot enough that the fuel burns as soon as it’s injected. They vary power by altering the amount of fuel injected, so the fuel-air ratio is much more variable than the fairly stable mixture in petrol engines. If you look at tractor pull competitions, they crank out incredible power from turbo/supercharged diesels by just turning up the boost and injecting massive amounts of fuel. Those things send a plume of black smoke up, the combustion efficiency doesn’t matter, it’s just a matter of pushing in as much air and fuel as the engine can withstand.

    Hydrogen burns at quite a high temperature if there’s a stoichiometric fuel-air ratio, so it causes NOx pollution. This can be reduced by using a leaner mixture, but you can’t burn as much fuel with a very lean mixture so the power output is reduced. Basically, once you go leaner than a stochiometric ratio, the engine power starts dropping for the same volume of air. If you have to run lean under full throttle to meet emission standards, you have to build a larger capacity engine to produce the same maximum power output. That larger engine will be heavier and more expensive to produce. Running lean to meet emission standards is a drawback, not an advantage.

    whatgoesup
    Full Member

    Nice description of the combustion processes above 🙂

    Re Gas engines – (either Natural Gas or Hydrogen – same broadly applies) can run at stoichiometric or very lean. At stoichiometric there are emissions issues so you’d need aftertreatment. Lean can avoid this need, but at the expense of increased cost of turbocharging (to avoid dropping power you increase the density of the air, so get more mass of air into the engine), a more expensive ignition system and “transients” (ability to cope with rapid changes in power or speed) become worse as you’re reliant on the turbocharger speed to drive enough air in to the engine to allow enough fuel without misfiring etc.
    You can’t just add more fuel to cope with transients like a Diesel can (within limits – usually imposed by black smoke creation) as a gas engine will “knock” which is a very bad thing.

    I expect that due to the above a passenger engine will run “as lean as possible” which actually won’t be that lean, so will still need aftertreatment.

    Note Diesel and Petrol can both run vey lean as well, and as time has progressed run leaner and leaner as technology has progressed to remove the limitations to doing so. As will all these things it’s a compromise between many factors. Emissions, power density, driveability, misfire etc.

    wobbliscott
    Free Member

    Petrol engines can run upto 34:1 AF ratio, but hydrogen could go upto 180:1 AF ratio. Whether or not it makes sense to run them at those lean rates in the real world is another matter, but similarly though petrol CAN run at upto 34:1 AF ratio doesn’t mean there are any production engines out there that do. The benefits of the higher AF ratio is that NOx can be addressed/reduced and they are easier to start…however downside is reduced power…but you don’t meed max power all the time so no reason why you couldn’t run super lean when cruising at low power levels bagging better economy and emissions, then as you demand more power the AF is changed to delver the greater power. Or even marrying an H2 ICE to electric in a hybrid so the engine needs to only ever run in a mode where it is minimising NOX and the electric motor boosts power.

    Appreciate that H2 engines might be larger and heavier to achieve similar power levels to petrol/diesel…but so what….the weight of electric motors and batteries in BEV’s is hardly the light weight solution. We’re never going to beat petrol and diesel engines so lets accept that (well not in the next hundred years at least). Petrol/diesel is just too good and convenient a fuel. So again…H2 engines don’t have to compete with petrol/diesel engines and power plants….they need to compete with BEV’s and whatever other alternative power plants there are out there. Provided there was a decent Hydrogen network out there then moving to hydrogen fuel cell or hydrogen ICE would be a much more familiar and convenient step to emissions free motoring for most people used to fossil fuelled cars. Future generations might develop different behaviours and drive different solutions – maybe even car free solutions…but thats decades in the future.

    whatgoesup
    Full Member

    As an end result of all the above I’d expect a production hydrogen engine to be broadly comparable to a comparable petrol engine in terms of power density, in terms of power density and need for aftertreatment.

    whatgoesup
    Full Member

    * Disclaimer – this topic is outside of my professional experience and so is now into just “my opinions”.

    There are some interesting papers on this from a quick google search.

    One interesting conclusion – port injection hydrogen about 83% of the power density of gasoline, using more advanced techs potentially around 115%. So broadly comparable.

    MDPI Applied Sciences paper
    Another paper

    thols2
    Free Member

    Another thing to consider is the direction that F1 engines have gone in. They drastically reduced the maximum fuel and fuel flow to encourage economy. The engines are turbocharged, with the turbo also able to drive an electric generator (so basically a petrol-electric turbo-compound). To some degree, the ICE functions as a gas generator for the turbo-electric generator. It’s hard to know how much of the public statements are just bullshitting to throw rivals off, but the Merc people said that they incorporated ideas from diesel combustion to improve combustion efficiency. Those engines are getting better than 50% thermal efficiency, which is impressive.

    Thing is, using the ICE as a gas generator means that you need to find the optimum level of energy going out the exhaust (i.e. is it better to extract the energy through the ICE or the turbo-generator?) Just bolting a turbo-generator unit onto an existing engine would not be optimal, you’d need to rethink the entire engine and combustion process.

    Those engines are fantastically expensive, but the basic concept might be applicable to things like HGVs which burn a lot of fuel and do massive annual mileages. Fueling them with a biodiesel/hydrogen mix might give better energy density than BEV tech and help with the NOx emission problem. Having them idle for 30 minutes while they stop to recharge batteries would be expensive down-time, so chemical fuels would avoid that. Not saying that’s what’ll happen, just that that’s probably the types of things that engineers will be looking at.

    molgrips
    Full Member

    Hydrogen burns at quite a high temperature if there’s a stoichiometric fuel-air ratio, so it causes NOx pollution.

    At stoichiometric there are emissions issues so you’d need aftertreatment. Lean can avoid this need

    Hang on. NOx emissions happen because there is ‘spare’ oxygen in the cylinder that hasn’t got fuel to react with so it reacts with nitrogen at high enough temps. So that seems to me that running lean would create more NOx and running a stoichiometric mixture wouldn’t have this problem. That’s why diesels create much more NOx, because there’s always hot air in there at some point and at light loads there’s a lot. That’s what EGR is for. Where am I wrong?

    molgrips
    Full Member

    To some degree, the ICE functions as a gas generator for the turbo-electric generator.

    This is what Toyota did with their first hybrid car. It’s both a series and a parallel hybrid in varying amounts depending on load and road speed. Very clever bit of kit. I don’t know if trucks use Toyota’s hybrid system or if they could, however.

    thols2
    Free Member

    So that seems to me that running lean would create more NOx and running a stoichiometric mixture wouldn’t have this problem.

    The combustion isn’t instantaneous (i.e. detonation, which destroys engines). In a petrol engine, the spark plug ignites the mixture in one place. The flame front spreads out and burns through the charge. Controlling the timing and geometry of this is a big part of combustion chamber design. Remember that the mixture is ignited before the piston reaches top dead center, so the combustion chamber is still getting smaller, then it expands as the piston starts on the power stroke. If the ignition timing is too advanced, you’ll get detonation. If it’s too retarded, you’ll lose power. At high revs, it needs to be advanced, at low revs, it needs less advance. Because the combustion isn’t instantaneous, you have nitrogen and oxygen mixed at high temperatures and pressures. If the combustion is too hot, some oxygen will combine with nitrogen instead of fuel and produce NOx.

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