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Old 06-25-2019, 07:23 PM
483 posts, read 204,083 times
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The Lenoir engine has a bad reputation for inefficiency. I think this is due to, first, the crudeness of the first examples, which were converted steam engines, and second, using the pressure-volume diagram to compare equal displacement engines. A Lenoir at 25% cutoff induces the same amount of fuel-air mixture as an Otto engine of 25% the displacement, so naturally an equal-displacement Otto engine will make four times the power, more or less. The Lenoir would run much cooler, there being no compression, and cleaner. It probably wouldn't need a catalytic converter since it wouldn't make nitrogen oxides. Governing it by cutoff would give a large range of torques available at any given speed. In fact, it would be as flexible as a steam engine -- it might even start on spark from stalled, under load. A clutch and small starter motor would probably be more reliable, though.
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Old 06-26-2019, 06:51 AM
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Well, a great deal of the fuel efficiency of an internal combustion engine comes from the amount of expansion. To get a similar expansion ratio of a 9:1 compression ratio Otto cycle, you're going to have to have one hell of a huge stroke. During that huge stroke you're going to be losing a lot of heat out of the cylinder.

Efficiency is not measured by displacement; so the argument of comparing engines of different displacements is a red herring. Efficiency is measured by power output divided by (consumption rate of fuel times energy content of the fuel). Basically at the most summarized level you need the maximum difference possible between the highest temperature of the cycle (basically, combustion temp. in the cylinder) and lowest temperature (basically, exhaust gas temp.) Turbo-supercharged engines, for example, do good work in extracting additional power from the hot exhaust gas; while the additional pre-compression of the intake charge also increases the upper temp. The very best efficiency is going to be achieved by running the engine as hot as possible without melting stuff. The limits on that are set by the ability to cool it.

As far as emissions, again, the more efficient the use of the energy content of the fuel, the fewer emissions other than water and CO2 there will be. Hotter operation reduces unburned hydrocarbons. For other specific pollutants you have to look at specifics of operation.

The most likely reasons Lenoir did not include a compression cycle in his design were:

1) For a long time, coming from steam engine practice, it was believed that a four stroke cycle would have difficulty being self-sustaining due to two strokes without power.

2) The state of metallurgy and mechanical design (especially con-rod bearings) couldn't support the mechanical forces involved in having a compression stroke. Note that it wasn't till after the second World War that common automobile engines universally adopted pressurized lubrication and compression ratios above about 5:1. Now go back to 1860 where the standard of design would have been external con rods and lubrication provided by a guy with an oil can periodically dripping oil into the big end and small end bearings: now imagine trying to keep a 9:1 compression ratio engine from flying apart with those kinds of bearings. Today's 11:1 or higher turbocharged engines with main and rod bearing clearances on the order of 0.001" and superfinished journals and three and four layer journal bearings, supplied with 15 psi filtered oil from a gerotor pump, would have been unthinkable.

But mechanical design doesn't trump thermodynamics. If you look at the development history of IC and external combustion engines as well as turbines, it's a continuous march toward higher temps and pressures at cycle max and lower temps and pressures at cycle min. It doesn't matter how well developed your 5% Carnot efficiency engine is, it's not going to outperform a 35% Carnot efficiency engine.
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Old 06-26-2019, 11:38 AM
Location: Aurora Denveralis
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Yes, there are a hundred basic different ways to get work force from combustible fuel.

About 95% of them are laboratory curiosities with no hope of reaching efficiencies or relative output useful in the real world. But each one seems to have its champion wondering why it hasn't taken over the world.
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Old 06-27-2019, 07:49 AM
483 posts, read 204,083 times
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Some crude calculations indicate that, at 25% cutoff, the peak pressure would be 7 bar and the exhaust would be at ambient pressure. However, since the exhaust is at four times the ambient specific volume, it would have four times the absolute temperature. Not a particularly cool-running engine. Efficiency, you put in 7 and throw away 4, so 3/7 or 42%. Theoretical. There's also the possibility of regeneration, which I understand is not included in the Carnot cycle and the Carnot efficiency limit.

MEP would be 3 bar, as compared to 10 bar for modern Otto-cycle engines. I assume the latter figure is the average over all four strokes, so the Lenoir being a two-stroke would be no advantage. The increased displacement necessary for the same power may be largely counteracted by the lighter construction for the lower pressures.
I suppose the next step is to convert a lawn-mower engine and see how it goes.
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