Introduction

Most small airplanes are powered by piston engines. Car engines are sometimes used for kit or experimental airplanes. It seems like a logical thing to do since most car engines are reliable and less expensive than aviation engines. Yet while some car engines have performed well in aviation, they are the exception that proves the general rule to the contrary.

Here I’ll discuss some of the important ways in which airplane engines are different from car engines.

Rotational Speed

A typical prop for a small airplane has about a 76″ diameter (more or less). That’s a circumference of about 20′, which is how far the tips move in each revolution. The speed of sound is about 1100′ per second (sea level standard conditions), so that’s 55 revolutions per second, which means at 3,300 RPM the tips of the prop are moving at the speed of sound.

When the tips move faster than about 85% of the speed of sound, they start to lose efficiency. The airflow changes and they start making more noise & turbulence, and less thrust. And it creates unnecessary stress on the prop. So we need to limit the prop to about 2800 RPM. But we need to limit a bit more than that, because the airplane flies at high altitude where air is colder and sound travels slower. So typical small airplanes like this have a prop redline of 2700 RPM, plus or minus (lower for bigger props).

Power moves the car, or the airplane, or anything else that moves. In an engine, power is torque * rotational speed. Cars have a transmission enabling the engine and wheels to spin at different speeds, so they can rev up the engine to make good power, then gear it down to the wheels to maximize performance. To avoid the complexity, weight and reliability issues of a geared engine, in most airplanes the prop is bolted directly to the engine crankshaft. Thus, limiting the engine to 2700 RPM limits the power it can produce.

Consequently, most aviation engines don’t make much power for their displacement (for example the popular Lycoming O-360 which makes 180 HP from 360 ci), but they are designed to produce their rated power continuously while being lightweight and reliable.

Duty Cycle

Cars spend a lot of time in traffic constantly changing speeds. And cars rarely use their full rated power, but spend most of their time producing only a small part of it. For example, it takes about 30 HP to move a typical car down the freeway at 60-70 mph. For a car with a 150 HP engine, that’s only 20% of its rated power. A car engine is optimized for this duty cycle: to be efficient and reliable while producing a low % of its rated power.

Airplanes spend most of their time in cruise flight moving at a constant speed. The engine is running at a constant speed at or near wide open throttle, producing a high percentage of its rated power. For example, cruising at 70% power is typical. Airplane engines are designed to operate efficiently and reliably while generating their full rated power.

Lightweight

The value of light weight in an airplane engine is obvious. Consider the Lycoming O-360 mentioned above. It is a large displacement 360 ci engine that weighs only 260 lbs. A typical car engine of similar displacement weighs more than twice as much.

Of course, that 360 ci car engine would produce more than 180 HP. So for a fair comparison consider a modern car engine making 180 HP, like the Mazda Skyactiv 2.5. It produces 180 HP and weighs 260 lbs. In power and weight it’s similar to the Lycoming. But that Mazda is not designed to produce its rated power continuously. If you ran it constantly at wide open throttle at 6000 RPM it would not last very long.

It’s not easy to produce a lightweight engine that can operate reliably while continuously producing its full rated power. From a power / weight / reliability perspective, the Lycoming O-360 is comparable to modern car engines in 2022. This is especially notable when one considers that the Lycoming is a design from the 1950s.

Efficiency

Modern car engines are fully computer controlled. The driver applies a certain amount of throttle, and the engine computer determines the valve timing, spark timing, air/fuel ratio, etc. and constantly changes or adapts these settings to the conditions.

Airplane engines are manual. The pilot sets the throttle, RPM, and mixture manually. How can a human compete with a computer? Pretty well, it turns out, because the airplane spends most of its time in cruise flight, running at a constant power level, RPM, and altitude. This gives the pilot time to carefully optimize these settings and leave them there for hours.

One way to measure efficiency is miles per gallon. That Mazda gets about 40 miles per gallon on the freeway. A Cessna 172 in cruise gets about 18 miles per gallon. The Mazda wins, right? Well, it’s not really a fair comparison because the Cessna is going twice as fast. If you drive that Mazda twice as fast (say 130 miles per hour), it’s going to get about 1/4 the fuel economy, which is 10 miles per gallon (or less). So at the same speed, the airplane is almost twice as efficient. Indeed, other airplanes like Mooneys are far more efficient than the Cessna.

Yet this method of measuring efficiency is more about air resistance or drag, than the engine. Airplanes are just inherently more efficient than cars. What if we ignore that and focus on the engine itself?

Another way to measure efficiency is BSFC: brake-specific fuel consumption. That is, how much fuel does the engine consume to do a certain amount of work? One way to measure this is horsepower per gallons per hour.

Let’s estimate this for the Mazda. Suppose it’s getting 40 miles per gallon at 65 miles per hour. Each hour it burns 65/40 = 1.625 gallons of gas. Traveling that fast takes about 30 horsepower, so it produces 30 / 1.625 = 18.46 HP per gallon per hour.

Now consider the Cessna 172. It’s getting 18 miles per gallon at 130 miles per hour. Each hour it burns 130/18 = 7.2 gallons of gas. But how much horsepower is it generating? That is about 65% power, which is .65 * 180 = 117 horsepower. It produces 117 / 7.2 = 16.25 HP per gallon per hour.

So here the Mazda engine is about 13% more efficient (18.46 versus 16.25). However, keep in mind that this is when producing only 30 / 180 = 17% of its rated power. The Lycoming was producing 65% of its rated power. When you open the throttle to make the Mazda produce 65% of its rated power, its efficiency drops significantly, well below the Lycoming.

Note that each engine, car or airplane, is more efficient than the other when operating within its typical duty cycle.

Reliability and Durability

If an aircraft engine fails in flight, the airplane stays in the air but not for long; it becomes a glider that is going to land somewhere very nearby, very soon (within minutes), and most likely off-airport. It is an emergency situation that can lead to injury or death. If a car engine fails, you coast down and simply pull over to the side of the road. It’s an inconvenience, not an emergency.

Airplane engines are designed for reliability. Their spark plugs are powered by magnetos, so (unlike a car) the engine keeps running even if the electrical system fails. Each piston has 2 spark plugs, so if one fails, the piston still produces power. They have 2 separate magnetos and half the plugs are fired by one magneto, half by the other, so if one magneto fails, the engine keeps running. They are air cooled, so there is no water pump that can fail, no radiator that can leak. Also, they spend most of their time in cruise operating around 2500 RPM, so they have static spark timing optimized for that speed – no need for timing advance means simplicity and reliability.

Plenty of historical examples demonstrate the problems using car engines in airplanes. In the 1980s, Mooney made a plane that was optionally powered by a Porsche engine. It had so many problems, the changes needed to make that engine reliable in aviation were so extensive, Porsche gave up and discontinued it. Thielert had a similar situation building Mercedes diesel engines for aviation use. You can google the details on these and other examples.

Yet how do we reconcile this history with the fact that aviation engines use technology that is more than half a century old? A pilot’s pet nickname for Lycoming is “Lycosaurus”!

Consider how any engine becomes reliable: start with a good design, then tweak a little it every year to address any problems discovered in the field. Cars follow this pattern. They come out with a new engine, the first year has some issues, each year it gets a little better, then 5-10 years down the road, just when the engine is reaching its peak, they scrap it and start all over with a new design incorporating new technology. Imagine how reliable car engines would be if they never scrapped it, stuck with the design and continued that incremental improvement for 50 years. The engine would be “low tech” for sure. And may not be as efficient. But reliable? You betcha – they’ve seen just about every failure there is and incorporated changes to address it.

This is what a typical Lycoming or Continental certified aircraft engine is: the result of more than 50 years of incremental improvement on a design that was pretty good to begin with. It’s ancient technology, yet it’s highly optimized and adapted in an incremental, evolutionary way.

Production

Last year, Mazda built more than half a million engines. Lycoming produced about 4,000 engines. Yet this difference of more than 100:1 understates the difference, because there are many car manufacturers while there are only two manufacturers of certified aircraft engines: Lycoming and Continental. For each aircraft engine built, more than 1,000 car engines are built.

To produce reliable engines at such low volumes, aircraft engine manufacturers use completely different production methods. Each engine and all the parts in it are individually hand-built, inspected, and tested before leaving the factory. Visit a modern car engine factory and it looks like a scene from a sci-fi movie where robots have taken over the world. Visit an aviation engine factory and it looks like you’ve gone back in time to a boutique hot rod custom engine building company.

Conclusion

Cars and airplanes are completely different applications with different requirements. It should be no surprise that engines optimized for one are not well suited to the other. High technology is not and end, it is a means to an end. The end or goal is meeting the requirements for the application. Pilots building their own kit / experimental airplanes can use any engine they want. Yet most of them still prefer certified aviation engines from Lycoming or Continental, despite the high cost and low technology compared to car engines. This is not irrational, but backed by some of the reasons discussed above.

All that said, much of the reason aviation engines are so low tech and expensive, is certification. The cost to certify an aircraft engine is so high, and production volume is so low, they can never break even on a new engine design. Over the years, this forced them to differentiate and improve their products with incremental tweaks to existing designs. One can view this as an unintended consequence of over-regulation: certification rules that were intended to promote safety, led to technological stagnation. Or, one can view it as a beneficial outcome that optimizes for reliability in their intended application, which is crucially important with aircraft engines.