Seriously, at least get your facts straight before you mislead people. There is no fun in that. Or change the topic. This is false, one of the reasons people don’t trust the internet. You came on here sounding like a “trusted source” but by all of your arguments and so-called ”logic” it is obvious you know jack-squat about engines and the effect of engine braking. Please stick to your day job (hopefully this is not it).

Thanks

Your amazing knowledge of the ins and outs of a modern motor vehicle engine shocks me, and i would love to know, where you have learnt this all from?

I’m currently studying motorsport engineering in college, in my third year of HND level 3, which i want to take to uni.

Could you possibly let me know where you’re studying and how its going etc?

Thanks very much, and once again, amazing!

I think what you mean by ‘compression ratio’ is actually the overall pressure ratio. What I mean by ‘compression ratio’ is the ratio between the combustion chamber volume when the piston is at bottom dead centre and when it is at top dead centre.

The pressure ratio is highly dependent on more than just compression ratio, as you point out. It also depends, for example, on atmospheric pressure or pressure inside the intake manifold, if the engine uses forced induction. And a host of other factors.

In my comprehension, one can speak of an engine having distinct and different compression and expansion ratios if that engine’s piston travel during the compression stroke (from piston BDC to TDC) is longer or shorter than the piston travel during its power stroke (from piston TDC to BDC) irrespective of the moment ignition or injection occurs or the intake or exhaust valves open.

Which, I think, is not the case with any automotive engine currently in production, though I may be wrong.

“The goal of the modern Atkinson cycle is to allow the pressure in the combustion chamber at the end of the power stroke to be equal to atmospheric pressure; when this occurs, all the available energy has been obtained from the combustion process.”

This is a bad goal. Nevertheless, if it’s attainable by closing the intake valves during the compression stroke then it’s surely attainable by not opening the throttle plate as much.

As I said before, I think a much more relevant indicator of engine efficiency are specific fuel consumption under part load as well as full load and exhaust gas temperature under various loads and rpm.

Peugeot are one of my favourite manufacturers but I don’t think this design is all that good.

The disadvantage of the four-stroke Atkinson-cycle engine versus the more common Otto-cycle engine is reduced power density. Because a smaller portion of the compression stroke is devoted to compressing the intake air, an Atkinson-cycle engine does not take in as much air as would a similarly designed and sized Otto-cycle engine.

Four-stroke engines of this type with this same type of intake valve motion but with a supercharger to make up for the loss of power density are known as Miller cycle engines.

If you were to close the throttle plate, wouldn’t that increase pumping losses and reduce overall efficiency?

Why don’t you just not open the throttle plate as much?

I would try a petrol engine with high compression ratio, something like 18:1, that would run choked (though still stoichiometric) through the throttle plate most of the time so as to avoid preignition or detonation. As rpms increased the throttle plate would be allowed to open to a greater extent so as to compensate for poorer cylinder filling.

“This system results in a longer power stroke compared to the compression stroke (or, another way of looking at it is that the peak pressure of compression is lowered and hence the pressure everywhere is lowered, and hence final pressure after the power stroke is closer to atmospheric pressure, hence more efficiency).”

As I see it, the power stroke isn’t any longer in terms of crank degrees.

Low compression pressure is not a good thing. Besides not homogenising the air-fuel mixture as well and making combustion slower (which promotes heat loss into the engine) it means you’re working against a steeper pressure gradient in a naturally aspirated engine as the reciprocating piston engine makes power from the pressure difference between the combustion chamber and the crankcase during the power stroke.

Restricting air flow into the cylinder during the induction stroke means the piston descends inside the cylinder against a pressure gradient. Part of this energy is probably recovered through less resistance during the compression stroke but energy is lost again during the power stroke, when the piston again descends. Some of the energy of combustion is expended first overcoming the negative pressure difference, before positive pressure difference (between the combustion chamber and crankcase) can start to push the piston down. This energy is not recovered during the exhaust stroke. This is what throttling losses are. And it’s one of the reasons why diesel engines usually have better specific fuel consumption, particularly at part throttle. Diesel engines are freebreathing. If they use a throttle it’s probably to promote and control exhaust gas recirculation into the intake manifold. Of course, throttling somewhat reduces pumping losses as well, but the net effect is one of decrease in efficiency.

I see no difference between not opening the throttle plate as much and closing the intake valves after compression commences. In fact, I think closing the intake valves later might be less efficient than not letting as much mixture inside the cylinder in the first place. But I may be wrong, or else Toyota would have opted for this approach instead?

In any case, in a naturally aspirated engine you’re always pumping against a pressure gradient. Even with no throttle at all, you’re still pumping against a pressure gradient because there is simply not enough time to completely fill the cylinder with air to atmospheric pressure. And this gets worse with rpm, which is why engines (naturally aspirated ones especially) benefit from variable intake valve timing and lift. But throttling makes things worse.

Also, I don’t think exhaust pressure is a good indicator of engine efficiency. A good indicator of engine thermal efficiency is exhaust gas temperature. I presume measured as close to the exhaust valve as possible. Though this does not account for thermal energy lost to the engine and dissipated through its cooling system.

Generally speaking, an engine with a higher compression ratio will be more thermally and mechanically efficient, because combustion gasses may expand more before being allowed out of the cylinder in addition to the advantages already espoused regarding . So there is more opportunity to extract useful work.

This is another important reason why diesel engines are more efficient, as they generally have higher compression ratios.

LMF, there are a plethora of problems innate to wankel engines, foremost of which are sealing and high combustion chamber surface to volume ratio, which is conducive to heat loss into the engine. Low compression, or rather expansion – in the case of the Atkinson cycle, ratio has traditionally also been a problem with the wankel. All this adds up to typically higher specific fuel consumption figures for the wankel.

Other problems include uneven and higher heat loss troughout the stator shell as only one side of it is exposed to combustion, and exposed continuously. Difficult lubrication of rotor seals. Difficult maintenance.

An Atkinson cycle engine based on the rotary concept would share problems with the Wankel. Designs that seem great on paper usually don’t work as great irl. I imagine sealing and lubrication would be even more of a problem for an Atkinson rotary than it is for the Wankel. The compound, intermeshed rotary adds a lot of complexity and great opportunities for wear, with its obvious lubrication difficulties. Possible balancing issues.

The reciprocating piston based Atkinson engine also doesn’t seem very promising, imho. Packaging is the obvious concern. But I also see problems with its conrod and levers gear as well. There’s more to bend and flex at rpm and more reciprocating mass to suck up useful work. Lubrication issues and small journals. And on top of all, engine balance issues, the same as with the offset cylinders engines.

If only there were some easy way to delay the moment of peak cylinder pressure closer to when the conrod is in the most favourable position to imprint torque to the crank without sacrificing compression ratio, engine balance or increasing reciprocating mass.