Don't know if it was the lowest but there was a turbo-compounded aircraft piston engine during WW2 that was down around .35 lb/hp-hr. I didn't make it into production far as I know.
The aircraft engine was the wright cyclone TC3350. It was produced till the early 60's for prop driven comercial airliners. Personally I think it is the best method (turbocompounding)of attempting to achieve fuel in/power out bsfc #s. on a gasoline fueled engine. I also think there is some merit to attempt to recover the heat lost in the cooling system. The wright engine was an air cooled radial. I believe a liquid cooled engine producing steam either directly from the coolant or via a heat exchanger should result in further fuel economy improvements. I am currently working on such a project. So far results have been very encouraging but fabrication and evaluation have been moving much more slowly then I had hoped.-----Phil
Most stratified charge engines revert to a homogeneous charge at WOT. (To get max power) The fuel economy gains are at part throttle.
If you look at the direct injected gasoline engines, the mileage figures based on testing in Japan will be much better than the normally fueled engines. This is because there is a lot of time at idle in the Japanese mileage drive cycle. There is little idling on the U.S. drive cycle and the direct injection (stratified charge engines all) engines do not show nearly as much gain in MPG when idling is not a big part of the program.
Since gasoline engines tend to show best BFSC at WOT and low speeds, stratified charge does not hold much advantage to my mind in the BFSC race.
Some really impressive stationary engines running natural gas are approaching 40% thermal efficiency on the Miller cycle. A high pressure turbo is used to perform some of the work of compression: The intake valve is held open longer than normal and exhaust energy is used to fill the cylinder with several atmospheres of pressure rather than relying solely on stored flywheel energy to compress the charge. This is practical if you have one speed and fairly continuous loads. (See some of the big Waukasha engines.)
The two stroke diesel aircraft engine mentioned above may have been the Napier Nomad of the late forties and early fifties.
You have the name of the turbo-compounded engine above. I think my old thermodynamics professor would refer to that engines as running on the 'Atkinson cycle' since it got a little extra shaft energy out of the exhaust after the piston reached BDC. (the exhaust turned a small gas turbine geared to the crank for additional hp and the exhaust was then direct towards the back of the plane and some extra thrust was claimed because of that as I recall.)
Ehudson, I'm aware that most strat charge engines go homog charge at wot, but do you (or anybody else) have any idea if BSFC improves (even slightly) at wot when stratified? Regardless of power output. The main advantage of strat charge is pumping losses are waaay better but, you are running rediculously lean and a bunch higher compression. Seems like that would count for something.
good pnt on the japanese, didn't know that, makes sense.
Trying to decide whether to design a turbo setup or a cylinder head for strat charge. Hmmmm...
I thought atkinson cycle simply referred to a cycle where the power stroke is longer than the compression. Wierd contraptions.
For EHUDSON- I can't locate the data on the Napier Nomad. There is some performance data on the Napier Sabre in a book called "Major Piston Aero Engines of World War II". Fuel economy was good. The engine was supercharged. The engine design was good for performance & economy but it was an expensive design. It had the elegant sleeve valves instead of poppet valves.
for jminahan: there is a picture of Mr. Atkinsons engine in the book "Internal Fire" by Lyle Cummins. It had an extra link between the connecting rod and the crankshaft.
I do not know of any stratified charge gasoline engines that remain stratified at WOT.
When I referred to the Atkinson cycle, I had in mind the P-V diagram rather than one of at least two complex crank and linkage configurations that Atkinson developed. (These of course are interesting in themselves and provide the answer to the trivia question "what single cylinder four stroke fires every crank revolution").
If we look at the typical idealized air cycle P-V diagram for an Otto cycle, with the peak pressure on the upper left, the exhaust appears on the lower right side as a vertical line where the pressure drops to atmosphere. The Atkinson cycle, instead of having the abrupt drop in cylinder pressure continues the pressure decrease (expansion) until it approaches atmosphic pressure.
It is this extended use of gas pressure after the Otto cycle exhaust valve normally opens that was described to me as the Atkinson CYCLE. If memeory serves, the Atkinson engine was developed in an effort to work around the Otto patent before said patent was eventually nullified.
The Curtis-Wright engine continues exhaust expansion in a turbine rather than by extension of the piston stroke. It nevertheless uses energy normally wasted in the Otto cycle.
Regarding the turbo or stratified charge engine question, I'm pretty sure you will see a real benefit from the turbo.
j2bprometheus:
The Sabre was a gasoline sleeve valve engine as you correctly note. The Nomad was a 2 stroke diesel. The reference link below suggests a BFSC of .345 lbs/hp-hr. for the Nomad #2.
