The car vs the airplane...

[VNugget]

There's the rub, max HP for just about any street car engine you could name is achieved well beyond 3000-4000rpm, especially in engines that have relatively equal hp to light airplane engines (140-160hp is typical in 2.0L engines, or around 120cid).

That was my point exactly. The original question was why do airplane engines have horsepower ratings of about half the displacement in cubic inches, while car engines have about twice that (meaning roughly equal HP and displacement).

Power = displacement * BMEP * RPM, right?

Well displacement and manifold pressure are the factors we're keeping constant (for naturallly aspirated engines), and the compressions ratios are about the same, so the only thing left is RPM. Car engines are rated at at least twice the RPM of aero engines, so they develop twice the power per dislpacement. Makes perfect sense, right?

 

[I.P. Freley]

I'm agreeing with most of what you say, but it's been a while since my recips class and I don't remember how BMEP relates to displacement and RPM... Since I don't remember what BMEP is aside from what it stands for! I'm guessing it has something to do with compression ratio, but my brain is farting reaaaal good right now.

The problem I see with the above is that modern auto engines are far more powerful than their more ancient brethren in cars, to the extent that a 3-liter auto engine in something as mundane as a Honda or Mazda sedan makes as much power as an engine of twice the displacement from thirty years ago, with lower-octane fuel and no commensurate bump in compression ratio. Of course they achieve it at a higher RPM... But not something as crazy as TWICE the RPM.

Obviously I've forgotten a lot about the basics of engines, at least as far as the math is concerned.

 

[VNugget]

It's not just modern engines, either. I watched a history channel show about muscle cars from the 50s-70s, and all of the engines they showed as examples had the same numbers for HP and displacement.

 

[Some guy]

The top speed in a car is limited by the gearing in the drive-train. That engine can only turn so fast before it becomes a pile of parts.

 

[Gatorman]

lol...who wants to pull out a 350, bolt on a prop and see if it will fly?

 

[coolyokeluke]

The problem I see with the above is that modern auto engines are far more powerful than their more ancient brethren in cars, to the extent that a 3-liter auto engine in something as mundane as a Honda or Mazda sedan makes as much power as an engine of twice the displacement from thirty years ago, with lower-octane fuel and no commensurate bump in compression ratio. Of course they achieve it at a higher RPM... But not something as crazy as TWICE the RPM.

Obviously I've forgotten a lot about the basics of engines, at least as far as the math is concerned.

Those smaller engines with higher operating RPM's would require reduction gears if affixed to a propeller. They don't make much torque at low RPM like the older, larger displacement car and airplane engines. So not having a gearbox is a weight saving issue as well as something else that could break in an airplane. I'm speculating here, but I think the idea of air/vs liquid cooling comes down to the same issues; weight and complexity. As far as sticking a 350 V8 on an airplane, lots of homebuilders have done it. One of the issues I always hear about with car engines in aircraft is that our large displacement low horsepower aircraft engines are designed to run flat-out, where car engines rarely and only briefly make maximum power. I'm all for advancing the state of aircraft engine technology, but there are reasons (one of which has sadly been liablity issues) that our engines have changed little in the last 50 years: relaiblity and weight.

 

[avbug]

As far as sticking a 350 V8 on an airplane,

ORENDA!

Making a comparison between automotive applications and aircraft applications is nonsensical.

Aircraft powerplants are limited in RPM by propeller constraints.

I've run piston engines in aircraft at much higher RPM's, especially automotive engines such as the Subary Stratus) in aircraft installations...but with very little gain in performance at higher revs.

 

[UnAnswerd]

lol...who wants to pull out a 350, bolt on a prop and see if it will fly?

I have a welding book that shows a finished engine mount used for the installation of a 4.3L V6 engine into an airplane. The 4.3 is essentially a 350 minus 2 cylinders.

I also remember seeing some kitplane with a Chevy 502 installed........

 

[TrafficInSight]

There was an article in a mid 80's issue of AOPA Pilot about a Globe Swift with a Buick 255 V-8 in it. It never caught on.

 

[I.P. Freley]

The top speed in a car is limited by the gearing in the drive-train. That engine can only turn so fast before it becomes a pile of parts.

Top speed on most cars is drag-limited, not gearing-limited, as in topping out at redline in top gear...

Many cars don't achieve top speed in their top gear, though. In Corvettes, at least the more modern ones with a 6spd, top speed can't be achieved in 6th because the gearing is too tall, but it can redline in 5th gear, which would be an example of what you said.