Theory: not just disabled cylinders, but a power/compound cylinder engine

[gone-ot]

...true, but can anyone cite a "fuel economic" jet engine? I doubt it!

...our R-3350's each burned about 100 gallons of 115/145 AVGAS per hour, which is almost half of what other, non-PRT engines, consumed.

...jet's go thru JP-fuel like 'frat freshmen go thru "free" beer.

[mechman600]

Originally Posted by dcb

Problem is extracting the additional heat energy without undue back-pressure (which makes the pistons not want to come back up).

I'm not sure if a bit more backpressure really matters. I used to think it does, but not anymore. Any modern diesel engine with a variable geometry turbocharger can have upwards of 50 psi (yes, 50 psi) of exhaust manifold pressure at full load. You would think 50 psi is ridiculously high, especially compared to a naturally aspirated engine with almost zero exhaust manifold pressure, but this pressure is miniscule when compared to the other pressures that make things "go." Combustion pressure can be 2800 psi +. Compression pressure can be 800 psi + at TDC at full load with 35 psi of boost. I'm not sure that a relatively small 50 psi of backpressure means a lot.

I think what killed the piston engine in the airplane wasn't a lack of power and efficiency, but unreliability, complexity, expensive maintenance, and the inability to do high altitude well.

[Bicycle Bob]

The turbo-compound Napier Nomad held the fuel-specific record for years, and is still ahead of straight turbines because the intermittent combustion allows a higher peak temperature, and with it, better Carnot efficiency numbers. It was just considered too complex.
One thing to consider with the automotive application is that when running at part throttle, the exhaust can be just returning to ambient pressure, leaving nothing for the turbo or second cylinder So this would work best on a very low-powered car. I don't think peak temperatures will be a problem in the second cylinder at any point.

[dcb]

Originally Posted by mechman600

I'm not sure if a bit more backpressure really matters. I used to think it does, but not anymore.

depends on what you call a bit, dont know what engine you are thinking of either, but if it is a 7 litre job, running @ 4000 rpm, putting out 50 PSI, with a bore/stroke of say 4 inches (keep me honest here):

it has to push 50 sq inches of piston area, at 50 psi for 4 inches every revolution

so thats about 2500 pounds of force, for 1/3 a foot or 840 foot pounds per rev

or about 3350000 foot pounds per minute @ 4000rpm, or over 100hp!!

Of course the intake is presumably pressurized too, so that would offset the losses by the ratio of intake PSI to exhaust PSI. i.e. if the intake is at 25psi, you would be spending a total of 50hp just on backpressure. If I figure this right.

[Piwoslaw]

Originally Posted by Bicycle Bob

So this would work best on a very low-powered car.

Or as a generator, intermittently cycling on @ max load/efficiency for charging.
But doesn't any turbo'ed engine's efficiency increase with load?

[mechman600]

Originally Posted by dcb

...dont know what engine you are thinking of either...

Every diesel engine with a VGT and EGR has high exhaust manifold pressure.
A VGT (Variable Geometry Turbocharger) is simple: when the ECM wants more boost, it closes down the volute, much like when you squeeze the end of garden hose. The pressure in the hose increases and the water sprays much farther. When more boost is required, the volute closes, creating a) backpressure, and b) high velocity exhaust that spins the turbine much faster, raising boost levels. VNTs (Variable Nozzle Turbocharger) are slightly different, changing the pitch of the volute fins, but the principle is the same: more restriction = more backpressure + more boost.

Also, backpressure in a EGR diesel isn't a nasty side effect of the VGT, but necessary for EGR to occur. Unlike a gas auto engine, diesel engines require EGR flow at full power where NOX production is extremely high. In order to jam recirculated exhaust into an intake manifold with high boost levels, the exhaust manifold pressure has to be higher than the boost pressure. If is wasn't, EGR flow would be backwards. In the 12-13L and 15L HD diesels, I typically see 40 psi of boost and around 50-55 psi of exhaust manifold pressure at max HP. And lots of EGR flow. This pressure difference between the "hot" and "cold" sides of the engine, combined with an EGR valve for control, is what causes EGR flow in a diesel engine.

Diesels require much more EGR flow than a gas engine to have the same effect, because diesel exhaust contains a higher % of oxygen (which aids combustion, not what you want), and a lower % of inert gas (which cools combustion, what you do want) in comparison. EGR levels in 2007 + diesels (without SCR) are as high as 30% of the intake charge!

I would say that the your calculations for a 7L engine are nifty on paper, but in a real working diesel engine with many things that affect many other things - the kind of things that only engineers know the real calculations for - they are pretty much meaningless. But, so is this entire thread unless someone (AKA myself) actually tries the theory in question.

[dcb]

lol, well psi is psi, and area is area, and since it is measured values the egr doesn't mean anything for these calculations. so you are looking at maybe 20hp in a 13L (large swag), that is to say if you reversed the pressures and put 50psi on the inlet and 40 on the outlet (or any 10psi differential) you could expect 20hp on the shaft then subtract for losses (VE/friction). Pretty far from zero meaning though.

I'm merely trying to point out that backpressure is a very real consideration, not something to be ostracised to the land of magic.

[dcb]

But to your original post, if this DD15 takes ~20hp to make 50hp, then it isn't free, but still a bargain.

[Peter7307]

Originally Posted by mechman600

I think what killed the piston engine in the airplane wasn't a lack of power and efficiency, but unreliability, complexity, expensive maintenance, and the inability to do high altitude well.

Agreed and in most cases piston engines were linked to propellors which limited the top speed of the aircraft whereas the axial flow gas turbine was well suited to both high speeds and high altitudes.

Peter.