Piston-engine efficiency at altitude vs. in ground effect

We’re watching The Spirit Of St. Louis (1956), and it got me wondering: Is it more efficient fuel-wise to fly at a cruising altitude, or in ground effect? As altitude increases, air density decreases; thus the fuel mixture is leaned and you burn less fuel. OTOH, flying IGE reduces drag so that flying requires less thrust to maintain the same airspeed. Less thrust, less fuel.

For a given RPM/torque/power output and a given air/fuel ratio, a spark-ignited piston engine will be more efficient at high altitude. This is primarily because you need to open the throttle more to get enough air to make that power, and that will reduce the throttling losses.

However, for a given aircraft speed, you use less power to cruise at high altitude than you do at low altitude, because of reduced form drag. In theory, your throttle position shouldn’t be much different between the two scenarios; it’s just that at high altitude, that same throttle setting means less air mass flow through the engine, correspondingly less fuel flow, and better miles per gallon.

Note also that a piston engine can be operated lean (and efficient) any time you don’t need max power from it, regardless of altitude. We could run our gasoline-engined cars lean and get much improved fuel economy, if we didn’t care about emissions - but alas, the three-way catalyst in the exhaust can’t do its job unless the mixture is right around stoichiometric, so we’re kinda screwed on that count. (Lean-burn is an important part of why diesels tend to be more efficient than gasoline engines, but they use different exhaust aftertreatment technology that doesn’t require a stoichiometric A/F ratio.)

The Wiki on ground effect vehicles says this:

Given similar hull size and power, and depending on its specific design, the lower lift-induced drag of a GEV, as compared to an aircraft of similar capacity, will improve its fuel efficiency and, up to a point, its speed.

So I think this kind of answers the question you were asking: for a given speed and cargo/passenger capacity, GEVs have lower total drag force. However, as that link points out, there are a lot of disadvantages that explain why GEVs haven’t become the norm.

I may be missing something but I thought piston engines were notorious for losing power at high altitudes (with a corresponding loss in efficiency). At high altitude the piston engine is still sucking down fuel at a given RPM but it has less power so therefore less efficiency. It is not moving as much for the same effort.

This is where turbochargers/superchargers came into being. They managed to put more air into the engine and thus maintain power at higher altitudes (IIRC turbochargers are more efficient but suffer from turbolag).

Then you move into turboprops which are basically a small(ish) jet engine attached to a propeller. Much faster that piston propellers and efficient at altitude but inefficient at lower altitudes. Not as efficient as a piston at lower levels though so…tradeoff.

Then you get to full-on jets. A jet so efficient at high altitude, not as efficient as a turboprop but can fly high and a lot faster.

Not sure how ground-effect vehicles fit in here but I am guessing they were very efficient (although the Ekranoplan used jets so…not sure).

For a given air/fuel ratio and RPM, a gasoline-fueled spark-ignited engine makes less power when you reduce the air density in the intake manifold, and more power when you increase the air density in the intake manifold.

You can reduce the density in the intake manifold in two different ways:

  • strangle the engine by closing a throttle plate upstream of the manifold, or

  • move the engine to higher altitude, reducing ambient density

Conversely, if you want more power out of your engine, you can move to a lower altitude, open the throttle, add a turbocharger, or all three.

A properly running engine manages the fuel flow rate to achieve a target air/fuel ratio regardless of the intake air mass flow rate. Simple carburetors tend to run richer when you move to higher altitude, or leaner when you move to lower altitude. This is not because they’re putting out a constant fuel mass flow rate regardless of the intake air mass flow rate; it’s just because they’re imperfect at compensating for intake air density. Modern fuel-injected engines are much better at maintaining the target air fuel ratio; they’re equipped with fuel injection maps, throttle position sensors, and ambient pressure sensors so they know, pretty close to the mark, how much fuel the injectors should squirt during each combustion cycle under various circumstances of RPM, throttle position, and ambient pressure. Final adjustment to injection quantity is made by a closed-loop system that monitors exhaust gas composition and adjusts future injection quantities accordingly. A vehicle with a fuel-injected gasoline engine can drive from sea level to the highest mountains, and it will maintain its ideal air/fuel ratio the entire way. Its max power output will decrease along the way, because it can’t get as much air into the cylinders on each cycle. But it’s not less efficient: because there’s less air mass coming in on each cycle, it puts in less fuel on each cycle.

TL,DR: A piston engine at high altitude makes less power because it’s moving less air AND less fuel, but it is not particularly less efficient.

The Wikipedia page I linked to upthread says that for a given speed and cargo/passenger capacity, as a general principle, a GEV gets better fuel economy than a conventional aircraft.

Note: I’m not talking about the efficiency of an aircraft at altitude vs. a GEV, nor an aircraft at ‘low altitude’ (which implies an altitude lower than cruising altitude, but out of ground effect). Machine_Elf gave a nice answer, but I already knew about the mixture at cruising altitude vs. the mixture at a lower, non-cruising altitude. What I’m wondering about is a given aircraft, say a Piper PA-28, flying in ground effect vs. the same aircraft at 9,500 feet.

OK. I got confused because you made mention of piston-engines and mixture, but it sounds like what you really want to know is how the overall drag of an aircraft differs at high altitude versus in ground effect.

For subsonic aircraft, drag has two main causes:

  • Form drag is just due to the general shape of the vehicle. It increases in proportion to ambient air density, and in proportion to the square of speed.

  • Lift-induced drag is a necessary effect of the wings making aerodynamic lift. It’s highest at stall speed, when the wing is operating at very high angle of attack.

A wing derives most of its lift due to the air over the upper surface of the wing being deflected downward as it leaves the trailing edge, but air on the underside of the wing also contributes a small portion of lift (by being similarly deflected downward). When flying in ground effect (altitude <= 1/2 the wingspan), the ground restricts the air on the underside of the wing from being pushed downward. This increases pressure on the underside of the wing, contributing additional lift. This means that the total amount of lift required to sustain flight can be developed at a lower angle of attack, reducing lift-induced drag and reducing the power requirement.

Whether flying in ground effect gives better fuel economy than flying at cruising altitude depends on the specifics, e.g. a plane designed to fly very fast at very high altitude will almost certainly get better MPG there. Commercial passenger jets can’t even safely achieve their normal cruising speed at low altitude; compared to 35,000 feet, form drag at sea level is higher by a factor of at least 3 and would do catastrophic damage. Looking at the Piper PA-28 specs, I see the service ceiling is 14,000 feet. Air density there is about 60% of sea level - so if you come down to sea level to fly in ground effect, your form drag will be 1/0.6 = 1.67 times what it was at the service ceiling. Can’t say for certain, but it would surprise me if ground effect caused a decrease in lift-induced drag that offset that gain.