11 replies to this topic

[Moon Tricky]

Well I haven't seen a new thread for a while so I thought I'd jump in with one... I've had this idea for a while now, let me run it by all you knowledgeable people.

In a Miller or Atkinson cycle engine you effectively shorten the intake stroke by leaving the intake port open during the first part of the compression stroke. This is a bit of wasted time where no useful work is really being done, even though it does increase the efficiency, it doesn't seem ideal. But what if you instead left the exhaust port open during the first part of the intake stroke? Supposing you also opened the intake port correspondingly later. Back pressure is suddenly our friend here, because it is acting on the piston again giving us a second power stroke! You could put an extra valve on the exhaust manifold to control the effect (close it towards the end of the exhaust stroke to trap enough pressure to act on the piston). Then when the intake port finally opens, we end up with both a lengthened power stroke and a shortened intake stroke. Add a supercharger to get as much air in as before. Although you also end up with exhaust gas in the mix - any severe disadvantages of that? I've heard of Exhaust Gas Recirculation already.

[imaginesix]

Had to read it a few times to get it. Makes sense, but what about using the delayed exhaust closure in place of existing EGR systems? With variable valve actuation it should be possible to change between full power and this new EGR mode for all the efficiency benefits you mention.

Though for DI engines it would negate the advantage of valve overlap that makes turbos such an appealing option.

[Greg Locock]

Just a quick question.

What thermodynamic advantage can be obtained by expanding some exhaust gas a bit more than usual, and then recompressing it again?

[Moon Tricky]

Hmm, but it's getting expanded again on the first power stroke, so it's being expanded twice and only compressed once.

I'm getting confused thinking about it now.

What you could also do, is open the intake (and the extra control valve) before the exhaust valve closes, and blow the exhaust out by the supercharger, so it would be operating partly as a two-stroke. As long as the fuel is injected after the exhaust closes it should be ok as the rest of the induction stroke would continue as normal.

[rms]

Isn't a 'Miller' cycle just marketing hype.
If it is not then all race engines are 'Miller' cycle engines with delayed inlet valve closure, up to 90deg after BDC. A supercharger just raises the absolute pressure the engine see's ie. 1 bar Abs or 1.5 bar Abs.

I just don't see any difference.

[malbear]

would you chaps be happy for me to explain the differences , similarities and advantages of the Beare head with the Miller and Atkinson cycles?

[imaginesix]

Seems to me the difference isn't in the valve timing it's in the philosophy. Atkinson cycle looks to maximize the quantity of air that goes into the cylinder during each combustion cycle, thereby improving power per unit of displacement. Miller seeks to extract the maximum power from every combustion stroke thereby improving power per unit of fuel.

Am I wrong?

[imaginesix]

Originally posted by Greg Locock
Just a quick question.

What thermodynamic advantage can be obtained by expanding some exhaust gas a bit more than usual, and then recompressing it again?

The advantage in my uneducated opinion is that expanding exhaust gas and then recompressing it again is easier than expanding intake air and compressing it.

[Moon Tricky]

Originally posted by imaginesix
Seems to me the difference isn't in the valve timing it's in the philosophy. Atkinson cycle looks to maximize the quantity of air that goes into the cylinder during each combustion cycle, thereby improving power per unit of displacement. Miller seeks to extract the maximum power from every combustion stroke thereby improving power per unit of fuel.

Am I wrong?

I thought Atkinson cycle reduced the amount of air going into the combustion chamber, and therefore also the amount of fuel. By reducing the combustion chamber volume an efficient compression ratio is restored. The increase in efficiency is down to the power stroke being effectively longer than the intake stroke so more of the exhaust pressure is used.

A Miller cycle engine adds a supercharger instead of reducing the size of the combustion chamber, restoring the compression ratio that way and getting as much air as normal into the cylinder. The increase in efficiency here is down to the fact that a twin screw has an easier time compressing the charge up to 2atm than the piston, which is better at compressing from 2atm up to 9 or 10atm. The result is more power for the same amount of fuel.

The disadvantage of both, as far as I'm concerned, is that during the portion of time when the intake valve is kept open during the compression stroke, no useful work is being done, even though the piston is moving and there is still plenty of exhaust pressure escaping to the atmosphere. I just can't help thinking there must be something else you could do with that time.

[Greg Locock]

Heywood discusses 'over expanded' engines (as he calls them), sufficiently, for most purposes. Certainly enough to render most fantasising unnecessary.

[Moon Tricky]

Originally posted by Greg Locock
Heywood discusses 'over expanded' engines (as he calls them), sufficiently, for most purposes. Certainly enough to render most fantasising unnecessary.

Well there goes my only hobby.

[Greg Locock]

Oh well, try girls instead.