I've never understood why piston powered plane engines burn such a large amount of fuel per time period than car engines.
Can anyone elaborate on this?
I know that a C172M will burn ~5-9+ gph at 2200-2550 RPM between 2k-12k feet based on the POH. This gives a decent idea of the fuel burn in a very narrow RPM range at various altitudes. According to the POH, she came with a Lycoming 0-320-E2D. Wikipedia states that the 0-320 provides 150-160HP using a 5.23l engine.
My car on the other hand has a 2l 4 cylinder engine. At 70mph in top gear @ ~2300rpm she gets about 30mpg for a calculated 2.33gph. To me highway speeds at constant RPM provides comparable engine conditions to that of cruise phase of flight.
5.23l / 2l = 2.61
2.61* 2.33mpg = ~6gph
Does that basic analysis hold? Is the larger displacement the main factor in fuel consumption difference between the two? I consider this figure of ~6gph close enough to the 5-10gph listed in the POH for such a back of the napkin calculation if the analysis is more or less correct.
This leads to a follow up question: Is there a reason for the large displacement and low HP? For comparison a 5.7l truck engine from the 70s output 250HP for a gain of 100HP for .47l additional displacement when compared to the Lycoming. I'm in no way very knowledgeable about engine's but am guessing the answer lies in engineering trade offs for making the engine lighter or out of a different material for aviation use or for higher compression ratios for the fuel. Any detailed input would be appreciated.
I know that Volkswagen and Subaru engines have been used for aircraft. Although my research has been very light, it still looks like the fuel consumption is in the same range.
Going back to my car vs the C172M, or the EcoTec vs Lycoming, 173hp@ 5500rpm for the EcoTec and assuming a (most probably false) linear fuel consumption based on RPM,
5500rpm /2300rpm = 2.39
2.39 * 2.33gph (@2300rpm) = 5.56gph
for comparable HP.
If my analysis provided here is close to the true answer, then I think I've cleared up my misunderstanding by writing out this question. However, I would like someone more knoowledgeable about such things (possibly an A&P who is also a gearhead) to pipe in and verify or enlighten my analysis.