Only because the engines themselves haven't evolved to handle higher manifold pressures. Lots of advancements in internal combustion engines, like combustion chamber and piston shapes that help manage the flame front, direct injection, wideband O2 sensors, are entirely unknown in certificated aircraft and still on the periphery even in experimental aviation.
Like, Mazda is running 14:1 compression ratios in gasoline engines, but 7:1 is considered normal for a naturally aspirated Lycoming and it'll often dip into the 6's for turbocharged versions.
Cessna has the best selling model of airplane of all time, the 172, which has sold around 45K units since its introduction in 1956.
Mazda sells that many cars in a typical 2 month period. It’s no surprise that there’s more non-recurring engineering investment in car engines than airplane engines.
I mean, okay? I get that. But it's not like the Cessna 172 is a $25k item either, it's now over $400k.
There have been tons of advances in general aviation, look at panel technology between now and 15 years ago. That's awesome! Let's do the same thing but for engines now. But it isn't going to happen as long as no one is pushing for it; we have the efficiency in automobiles we do now because regulators have been pushing for better fuel consumption and lower emissions for decades. It'd be great if there was similar pressure applied to aviation.
I'd love to have a 172 that had no mixture knob and no issues with hot starts. Man, wouldn't that be awesome? This is what we should be asking for, not apologizing for why we're stuck with mechanical FI, carb heat, and shock cooling worries.
If Mazda spends $25M in NRE on a new engine program and sells that design for 8 model years, the amortized NRE is around $10 per car or about 3 basis points of the product.
If Lycoming or Continental spend $25M on a new engine program and sell them for 15 years in all 172s, it’s around $22K per airplane or around 550 basis points. When investments in engine development are over 180x more expensive on a percent of sales basis and over 2000x as expensive on a per-unit basis, it’s unsurprising that few companies are stepping up to do that development.
I don’t mind at all if you want to lobby for it; I think there are improvements that could be made, but I think you’ll find the economics daunting and the current engine tech to be pretty well matched to the application.
Regulation is exactly the reason why there are few technology improvements. Improving the engines is pretty trivial. Heck, Diamond adapted car engines for their aircraft.
_Certifying_ them is hilariously expensive for the manufacturers and users.
One can certainly fix that, but that's how it is. Main reason why the planes that consume automotive gas the most are in the 'experimental' category.
It's not just engine evolution, but TEL was replaced by non-lead additives, not removed. And the fuel mix impacts values critical for aviation but mostly irrelevant to cars, like fuel vaporisation for given temperature/pressure (this is AFAIK major issue with ethanol added to fuel and source of limits on use of MOGAS in aircraft) as well as impact in performance which are pretty much impossible to notice for a car (except maybe if you have engine with sparkplugs designed for leaded fuel) but matter of life and death in airplane.
Like, Mazda is running 14:1 compression ratios in gasoline engines, but 7:1 is considered normal for a naturally aspirated Lycoming and it'll often dip into the 6's for turbocharged versions.