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Gasoline engine efficiency

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LAN_403

Dr. Dark Current

Thu Apr 04 2013, 10:57PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Hi,
I was wondering, does the point (RPM) of gas engine maximum efficiency depend on required power, or not really? Let's say I need just a little power to support constant speed of a car, should I use a higher gear to bring the RPM low (lets say under 2k), or there is no real benefit in doing so?

HV Enthusiast

Thu Apr 04 2013, 11:07PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Lower RPMs means less parasitic losses. The higher speed your engine has to crank for a given load, the more parasitic losses there will be - frictional losses in engine, frictional losses in transmission, rotational losses for spinning / moving parts, loads such as the alternators, water pump, etc...

Ash Small

Thu Apr 04 2013, 11:45PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

big engine, low RPM......unless you want acceleration, in which case, it's 'as fast as you can burn the fuel'.....

Fitting a smaller carburettor used to be the way to achieve this.

klugesmith

Thu Apr 04 2013, 11:47PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Also, with lower RPM the internal air flow is not as lossy,
and chamber pressures are higher which makes the Otto cycle more efficient.

I think the only thing you gain by running in a lower gear & keeping the RPM higher
is wheel torque and power, for immediate acceleration without having to downshift.
That's why automatic transmissions generally shift down when you stomp on the pedal.

That said, I'm convinced that there would be fewer car wrecks if all cars had less power than they do now.
There are cases where drivers use power to avoid accidents.
But statistically, overpowered cars are more likely to be involved in accidents.

...

Fri Apr 05 2013, 02:02AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

There is of course an optimum speed for a given requested power output. This can be concluded from the simple argument that the efficiency of the engine is 0 at stall (0 rpm) and is quite low at the engines redline (maximum safe operating speed). For most car engines the optimum speed is 2000-3000rpm, so the usual advice is to shift to the highest gear you have when cruising at 50mph+ (and things like overdrive were created to further improve fuel efficiency by giving even lower gear ratios)

Carbon_Rod

Fri Apr 05 2013, 02:16AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

Every combustion engine has a tunable range where optimal performance is observed. In the case of gasoline engines, the constant acceptable rpm range is wider than diesel engines. However, the timing and cylinder pressures vary widely depending on the engine and intended fuel. A general assumption that tends to hold, is â€œfor piston based engines you can expect 1 to 2 hp/lb, and for turbines around 3 to 6 hp/lbâ€.

People often mistakenly assume better efficiencies will occur with higher grade fuels, but more often than not it will certainly damage the engine in the long term. The energy density per liter of fuel does vary widely depending on the ratio of organic molecular structures, and seasonal temperature. For example, fuels with methanol and ethanol may not perform well in some engines, while in other systems the difference is negligible. More notable, is a fuel like â€œPropaneâ€ which tends to vary widely in the ratios of the actual molecular form of the alkane Propane to other forms of C3H8. On some dual-fuel conversion systems, one can often observe around 10% power loss to traditional petrol in the same engine.

Anecdotally speaking, I was surprised to find a B80 bio-diesel mix tends to prefer running a little hotter than traditional diesel, and for highway travel gets better mileage. However, I still would never add a cetane booster to this type of fuel even if the engine is more likely to survive than a gas setup.

Cheers,

Steve Conner

Fri Apr 05 2013, 10:12AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yes. Power is the product of RPM and torque. For a given amount of power, a combustion engine will be more efficient at low RPM and high torque.

One exception is with highly-tuned gasoline engines like you might find in sports cars. These kinds of engines often use a high compression ratio to get more power. But this requires the ignition timing to be retarded at high torque/low RPM so the engine can use fuel of a reasonable octane rating without detonation. The retarded timing basically wastes energy down the exhaust. So engines like this may give their best fuel economy at somewhat higher RPM than a standard car engine. Then again, if you cared a lot about fuel economy you presumably wouldn't have bought a sports car in the first place.

Adding a turbocharger gives more freedom in the design. The ignition timing can be left at the optimum setting and detonation is controlled by limiting the boost pressure. Friction losses are reduced because the whole engine becomes smaller for a given power output. You can get lots of torque at low RPM, great fuel economy, silly amounts of power, and last but not least a "Turbo" badge

Sulaiman

Fri Apr 05 2013, 12:11PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

There is thermodynamic efficiency where each engine has an optimal range of rpm x torque
for a static engine it is normally (low rpm)x(high torque)
For traction/mobile the weight of the engine, gearbox and cooling system is important if the vehicle accelerates/decelerates or climbs a hill, in which case a smaller, lighter engine running at high rpm is more efficient.

For best efficiency in a static engine, use a Stirling engine.

Dr. Dark Current

Sat Apr 06 2013, 08:46PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Thanks for the replies guys

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