Atkinson vs. Otto Cycle

[Flying White Dutchman]

nice topic!

good info

[bredekamp]

OK, thanks.

Originally Posted by kohnen

Easy.

Efficiency = work out / fuel consumed

Power = work out / time

So, if you had an engine that puts out only 1/2 the power, but uses 1/3 the fuel, then the engine would be MORE efficient but LESS powerful.

[David Beale]

You had to ask how it could be more efficient.

If you are, say, racing (even from stoplight to stoplight as most people do), you want lots of power. So you tune the Otto cycle for max. HP.

In order to achieve this you end up running high compression (for technical reasons ) and this results in increased temps in the combustion chamber. So you must deal with this before it heats a piston enough to weaken it so the pressure will punch a hole through it. Yup, it happens. More often than many racers would admit (interviewer-"Whats the reason you're out of the race?" driver- "I burned a piston"). How do you cool a piston (and valves, which I didn't even mention)? Well, injecting a little more fuel than you actually need is one way that is usually used. Result, lower efficiency.


Another thing that results from the higher compression is higher friction between the piston and cylinder walls. Power loss and lower efficiency.

Engine power output (HP) is speced in torque per unit time per distance. Increase torque per second and you increase HP. So run the engine at higher RPM (adjusting the internals as required to live). More power strokes per second results in higher HP. You then end up dumping exhaust that is still burning, wasting yet more energy. You have to because a long stroke would self destruct at high RPM.

I could go on, but the result is what they are trying to achieve, that is, more HP. The side "benefit" is lower fuel economy, but you have to "pay to play" (a common phrase in the racing and car nut community - for obvious reasons). So they are happy with the result. We would not be.

There are many trade offs in engineering. Most are not fun.

[Qlara]

What you mention is called "Fuel Enrichment".

A conventional engine mixes air and fuel in the proportion that will burn up all the fuel using all the oxygen in the air. This is called the stoichiometric mixture and the ratio of air to fuel is approximately 14.7 to 1.

Modern cars maintain this correct mixture using a Mass Airflow Sensor in the intake manifold and residual oxygen sensors in the exhaust. If the air to fuel ratio increases, so that an excess of air enters the engine, the mixture is said to be "Lean". Unburned oxygen in the exhaust does no harm itself, but a lean mixture tends to burn HOT and can damage an engine not designed for it. The high temperature can also cause nitrogen to combine with unburned oxygen and produce oxides of nitrogen that contribute to pollution.

Conventional engines are biased towards a Rich mixture (ratio less than 14.7 to 1) when power demand is high. This "Fuel Enrichment" makes sure every scrap of air drawn into the engine is used up to get the maximum possible torque. The unburned fuel can be oxidized, up to a point, by the catalytic converter, but its energy is wasted resulting in lower efficiency.

[qbee42]

Originally Posted by Qlara

What you mention is called "Fuel Enrichment".

Conventional engines are biased towards a Rich mixture (ratio less than 14.7 to 1) when power demand is high. This "Fuel Enrichment" makes sure every scrap of air drawn into the engine is used up to get the maximum possible torque. The unburned fuel can be oxidized, up to a point, by the catalytic converter, but its energy is wasted resulting in lower efficiency.

This is on the opposite end of the spectrum from diesel engines, which always pull in the same amount of air for every cycle regardless of the amount of fuel. They have no throttle. To get more power, more fuel is injected during the power stroke.

Tom

[rposton]

A 1500cc Atkinson also uses less air than a 1500cc Otto Cycle engine. Less air in, less air out.

[bac]

Originally Posted by ksstathead

See also...

nice graphical treatment of the subject (& other engine types) at

Keveney.com

Very very nice! Here are the direct links to both engine types:

Animated Engines, Atkinson

Animated Engines, Otto Four Stroke

... Brad

[FL_Prius_Driver]

Don't forget to examine the rotary Atkinson engine at

Rotary Engine - White-Smoke

[CalvinL]

Originally Posted by FL_Prius_Driver

Don't forget to examine the rotary Atkinson engine at

Rotary Engine - White-Smoke

Ah this is what I was looking for - a hybrid using the rotary/wankel concept and the atkinson fuel cycle. Ahhh thermodynamics - can't get past those pesky laws.

[Wa1hog]

Does the Atkinson have the same style starter as a Otto? Does it require the same amps from the battery? If so then what battery starts the ICE in the Prius. Seems that it is an instant start?