Aircraft piston engines generally require priming, but automobile engines don't seem to need it. Why doesn't cranking the engine with the starter pull in fuel mixture that would ignite without help of the priming?
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7$\begingroup$ All carb engines needs priming after it's been sitting for a while and the carb has dried up. Right now very few cars uses carbs and even when carbs were popular on cars, most cars never sit so long to have a dry carb. However, it's not entirely correct that fuel injected cars doesn't need priming, it primes every time after you turn on the electrics (the light noise you hear when the speedometer sweeps). $\endgroup$– user3528438Commented Jun 9, 2020 at 6:01
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6$\begingroup$ Remember the choke in older cars? Of course do car engines start better with priming, but automatic control has taken that off your hands and made it invisible. Another sign that aircraft piston engine technology has stood still in time. $\endgroup$– Peter KämpfCommented Jun 9, 2020 at 8:03
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3$\begingroup$ @PeterKämpf The choke is more like the mixture than priming, though. The choke just richens the mixture to make it easier to start when cold. Agree with the sentiment though $\endgroup$– DanCommented Jun 9, 2020 at 9:21
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2$\begingroup$ @Peter Kämpf: Yes, and in the days between actual hand chokes and EFI, you'd pump the gas pedal a few times to squirt some extra fuel into the engine (via the accelerator pump). And on really cold days you might have to prime it with a few squirts of starting fluid. $\endgroup$– jamesqfCommented Jun 9, 2020 at 16:36
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$\begingroup$ @user3528438: I don't know what you mean by "when the speedometer sweeps". Maybe that's something specific to your particular car? But I strongly suspect that the noise you're hearing isn't the engine being primed, it's the electric fuel pump building pressure in the system. "Priming" would be controlled by the EFI system, and wouldn't happen until the engine cranks. $\endgroup$– jamesqfCommented Jun 10, 2020 at 18:59
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3 Answers
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Modern car engines are really very sophisticated compared to most airplane piston engines, which are 70 year old technology.
Modern cars typically have an electric fuel pump in the tank which is often started by opening the drivers side door, by the time your butt is in the seat the fuel system is pressurized. When the ignition system is activated the ECU (engine computer) looks at engine coolant temperature and outside air temperature and directs the electronic fuel injectors to meter the exact amount of fuel which is correct for the conditions, directly into the inlet valves. A cold engine needs a much higher ratio of fuel to air when starting (2-3 times when hot), which reduces as it heats. When it starts running the ECU also takes into account the mass of the air flow and oxygen content of the exhaust to maintain the ideal fuel-air balance for the smooth and efficient running of the engine.
This technology does exist for airplane engines, it's just expensive to fit so very few airplanes have it. Most light airplanes have very simple, completely manual systems. You shut down the engine by cutting off the fuel supply (closing the mixture), this ensures that the engine is completely dry of fuel so that if the prop is spun there's nothing in the cylinders which could go foom and spin the prop, potentially maiming someone.
Piston engines create a lot of suction as they operate, and rely heavily on vacuum suction to operate. The low pressure in the carburetor atomizes fuel, which then gets pulled into the cylinder, explodes which in turn pulls more fuel and air in. However, because the mixture was cut off the engine and carb are all dry (any fuel left in the carb bowl will evaporate over time), which produces little suction. No suction, no atomization of fuel, no foom. The primer pumps a very rich atomized mist of fuel into the intake for each cylinder, giving the engine something to foom, creating suction to pull fuel through the carb and sustain the engine. This is why you feel resistance when using the primer, you are forcing it through a bunch of atomizing nozzles. This is why you need to start right after priming, if you wait too long you lose atomization.
Note that many airplane piston engine carbs have accelerator pumps which jet raw fuel into the carb on quick throttle advances, this is to prevent a lean cut-off. Some people think it's the same as using the primer, but it isn't at all. Because there's no suction when you pump the throttle forward all you are doing is jetting liquid fuel into the carb, there's no vacuum to atomize it, so all you are doing is filling cylinders with raw fuel and they won't go bang. A single jet is good because having fuel in the carb bowl gives something to atomize once it catches, but too much will just flood the engine.
A hot start is very different from a cold start, the engine doesn't need as rich a mixture and the carb bowl still has fuel in it, so when you crank it you don't need to prime in most cases.
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1$\begingroup$ I always think of F words when I think how old the technology in my airplane engine is @DeanF. $\endgroup$– GdDCommented Jun 9, 2020 at 15:32
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1$\begingroup$ I also understand pumping the throttle instead of priming can cause fires. $\endgroup$ Commented Jun 9, 2020 at 16:16
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1$\begingroup$ Downvoted because the carb is not dry after idle cut off or when starting the engine. Also downvoted because the answer makes no mention of cold engines requiring an enriched air fuel mixture when starting. $\endgroup$ Commented Jun 9, 2020 at 16:26
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2$\begingroup$ Fuel does not explode in an engine, it burns. $\endgroup$– J WCommented Jun 9, 2020 at 18:28
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1$\begingroup$ The carb bowl will typically be dry in a typical cold-start @MikeSowsun, I've edited to make that clear. I have also emphasized the need for a rich mixture. $\endgroup$– GdDCommented Jun 10, 2020 at 7:33
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Aircraft piston engines require fuel priming or fuel enrichment only when cold. This is true for ALL piston engines.
Fuel enrichment is needed because the cold temperature does not allow the fuel to properly atomize/vaporize in the air-fuel mixture and may actually condense back into raw fuel on the cold interior surfaces of the engine. Fuel enrichment ensures that will be a proper air fuel moisture reaching the cylinders for starting.
In a carbureted automotive engine, fuel enrichment is done with a manual or automatic choke.
Typical carbureted aircraft piston engines do not use a choke but instead use priming lines to supply addition fuel enrichment.
On a four cylinder aircraft engine there can be one, two, three, or four priming lines which spray atomized fuel directly into the engine cylinder very close to the intake valve. These priming lines are typically 1/8” copper tubing. It is easy to over prime the engine, and some pilots prefer not having a primer line for each cylinder. Having fewer priming lines allows one or two cylinders to fire even when the others are over primed.
Here is a photo showing 1/8” copper priming lines connected to the engine near the intake valves. 
answered Jun 9, 2020 at 16:09
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1$\begingroup$ Hm, I remember that 30 years ago the car did indeed have an enrichment lever that you'd pull if it wasn't starting and try with that (and then it was sluggish if you forgot it on, because the mixture was too rich). $\endgroup$ Commented Jun 10, 2020 at 9:56
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1$\begingroup$ Yes, in the UK, i believe it was called a "choke" $\endgroup$– OwlCommented Jun 10, 2020 at 14:58
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Why we need enrichment in the first place? The gasoline is liquid. The ignition requires a mixture of air and gasoline vapour.
In a running engine, having enough air inlet speed, gasoline becomes mist that evaporates when the compression happens in the cylinder. The engine being hot also helps a lot.
When cranking a cold engine, the air speed is not enough to create a mist, you get larger droplets that are quick to reach the inlet and combustion chamber walls and settle there. The compression is weaker, so is the heating because of the compression. No enough gasoline vapour - no ignition. The solution? Add more gasoline to the wannabe mixture that refuses to mix.
Carburetted car engines (~30 years ago) had the choke valve that reduced the pressure in the carburettor, alowing more gasoline to be drawn into the air stream. The choke valve is a bit of reliability concern - it has to be able to almost completely close the air inlet, or it won't work. I think that's why it is generally not used in motorcycles and probably in aircraft as well. A choke valve not returning completely was a common failure mode and one won't notice until the full power is needed.
One still needs some enrichment in order to start the engine. In the aircraft, one wants a method that has rather low probability of killing the engine in mid-air. So we have priming.
And yes, car engines get priming, too. In modern injection-fed computer-controlled engines, an automatic single injection pulse before starting does the trick. Depending on the conditions, the ECU may use priming, starting enrichment (longer injection times when cranking), both, or nothing.
And I am sure I did (as well as almost any advanced driver) manual priming (remove the air filter and pour some gas directly into the air inlet of the carb) on many cars that refused to start in a cold weather a decade or two ago.
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$\begingroup$ "one won't notice until the full power is needed.". Which is exactly what you need at takeoff, and you'd be very likely to notice before you reach V1 (if indeed you even can at severely reduced engine power). On aircraft this is probably less of a safety concern than in other applications. $\endgroup$ Commented Jun 10, 2020 at 17:09
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$\begingroup$ A piston engine is a constant-volume pump, so the compression clearly does not depend on rotation speed (for normally aspirated engine; it does for the turbocharger). $\endgroup$ Commented Jun 13, 2020 at 18:29
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$\begingroup$ @JanHudec An ideal piston engine is a constant-volume pump. Real pistons and valves have leaks and these leaks become significant at low rpm and also tend to increase with hours/mileage. That's an important (but not the only) reason why older engines are harder to start. $\endgroup$– fraxinusCommented Jun 13, 2020 at 18:46