For a gasoline internal-combustion engine, used in accordance with the manufacturer's instructions (specified quality oil, fuel, etc. used), what changes the efficiency of the motor?

For example at higher RPM, more heat generated due to friction would mean it isn't as efficient as at lower RPM, since some energy is lost as heat.

The engine's alignment would also have an impact on the efficiency of the engine – for example if a car's motor is mounted upside down (i.e., 180° rotated from the camshaft's axis of rotation) it won't be as efficient as it would be the right side up. So there is some relationship between engine tilt/misalignment and efficiency.

Another factor could be the temperature: hot engine vs cold engine vs overheating engine. Also does ambient temperature affect the efficiency – e.g., if the same engine is running in Antarctica vs the Sahara desert?

What other factors might have an influence on the engine's efficiency (disregarding fuel types E10, U95, U98, etc.), and are they also valid for diesel motors?

I've been looking for references with Google's aid, but I haven't found anything that is conclusive.


1 Answer 1


Presumably we are talking about efficiency in terms of shaft/mechanical output energy as a fraction of chemical energy in the fuel being burned.

The most important thing to note is that piston engines are most efficient in practice when run at full design load. At lower loads they are less efficient because (to first order) pumping and friction losses are constant with each stroke, but power output is reduced. (I.e., you could say that they aren't making "full use" of the combustion chamber volume, but they are incurring the full costs of cycling it.)

On the remaining factors I'm mostly making educated guesses:

  1. Again noting that pumping and friction losses are constant, in general one would assume that higher air density will produce higher efficiency since it produces more power output per cycle. So to first order the engine would be most efficient running in cold air at low altitudes.
  2. Assuming the engine is responsible for its own cooling presumably it will lose less energy to cooling when run at low RPMs and in a cold environment where, in the limit, it requires no active cooling at all.
  3. Cold air also allows for higher compression ratios, which increase efficiency.
  4. Overheating is bad: Once an engine gets critically hot it has to retard timing to avoid detonation, which reduces both power output per stroke and compression. And in the limit, even assuming the lubricant doesn't experience temperature degradation, an overheating engine will experienced increased friction as moving parts expand and the metal begins to become sticky as it approaches its melting point.
  5. Too cold can be bad: When the engine is cold it may run at reduced efficiency only because the friction is higher until lubricants reach their minimum design operating temperature.
  6. Airflow turbulence and friction become more significant at high engine RPMs. So holding all else equal I suspect more efficiency can be extracted from lower-RPM piston engines.
  7. I would question whether the engine's orientation is a factor for fuel-injected gas engines – especially for direct-injection engines. If the lubricant system isn't designed to work upside down of course you have a problem, but otherwise I think the combustion cycle happens at high enough pressure that gravity isn't a factor.

Note that a lot of the factors that sap efficiency disappear if an engine can be tuned for and run at a fixed load in a stable atmosphere. Requirements like responsive, variable power output in wide temperature ranges require a lot of efficiency tradeoffs.

  • $\begingroup$ To comment on your educated guesses: I'm not sure about air density improving efficiency. First of all higher air density allows more air and fuel in a single stroke, increasing the power output but also the fuel usage. Stuff like ambient temperature and higher compression ratios affect engine design, but nearly all car engines have a fixed compression ratio, and I'm not sure how adaptive cooling is in current engines, so for a given engine those probably don't matter much. $\endgroup$
    – JanKanis
    Commented Apr 6, 2018 at 18:33
  • $\begingroup$ Overheating is bad, but the temperatures that melt steel are much higher than what engines reach. What happens first is the steel starting to deform plastically very slowly. $\endgroup$
    – JanKanis
    Commented Apr 6, 2018 at 18:48
  • $\begingroup$ Too cold (when the engine has only been running a short time) is mainly bad because the burning fuel mixture is cooled down near the cylinder walls and then doesn't burn as completely. Also the hot combustion gases cool faster while pushing the piston down, this decreases the pressure and thus the extracted power. $\endgroup$
    – JanKanis
    Commented Apr 6, 2018 at 18:53

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