# At what speed will the crankshaft be rotating if each cylinder of a four-stroke cycle engine is to be fired 200 times a minute?

Well the answer is 400rpm ; but I don't understand how.

Here's my thought process:

4 stroke engine would mean every piston is in a different stage (intake, compression, power, exhaust)

Which means 200 power strokes every minute which would suggest 200rpm.

Could you tell me where I went wrong? Thanks in advance.

• It doesn't mean every piston is in a different stage. It does mean that the piston is firing once every two rotations though. Think of how a four stroke works and how many full rotations it takes to get to the power stroke. Dec 9, 2021 at 11:48
• In your third paragraph, I think you're getting "4 stroke" confused with "4 cylinder". In a 4 stroke 4 cylinder engine, then, yes, each cylinder would be in a different stroke. (Note that the same would be true of a 2 stroke 2 cylinder engine.) But if the number of strokes doesn't equal the number of cylinders, that's not going to be true. Dec 9, 2021 at 20:53
• Careful it is not a trick question. Some four stoke engines spark (or fire) at the top of the exhaust stroke, especially on motorcycles and marine engines. This is know as wasted spark and may or may not be a red herring if this is an interview question. Dec 10, 2021 at 2:19
• This is scarily similar to the old jokes about doubling the oven temperature to halve the baking time Dec 10, 2021 at 14:17
• I'm not sure how this is an aviation question. This is strictly a question about internal combustion engines.
– J...
Dec 10, 2021 at 14:46

In a four-stroke engine, each cylinder goes through the following stages during one cycle:

(image modified from FAA Pilot’s Handbook of Aeronautical Knowledge)

Let's look at the motion of the piston and the crankshaft during each stage:

• Intake: the piston is moving down and the crankshaft performs half a rotation
• Compression: the piston is moving up and the crankshaft performs half a rotation
• Power: the piston is moving down and the crankshaft performs half a rotation
• Exhaust: the piston is moving up and the crankshaft performs half a rotation

So during one cycle, the piston has moved down and up twice and the crankshaft has performed 2 full rotations. Therefore, when one cylinder is fired 200 times, the crankshaft will have rotated 400 times.

• Ah, the good ol' "suck, squeeze, bang, blow"... Dec 9, 2021 at 16:28
• Note that in half the strokes, the engine is doing work on the crankshaft, but in half the crankshaft is doing work on the engine (the power stroke is so much more than the other ones that the net work done on the crankshaft by the engine is positive). So 200 firings per minute means 200 rotations' worth of the engine doing work on the crankshaft, and 200 rotations' worth of the crankshaft doing work on the engine. Dec 10, 2021 at 3:43
• @Acccumulation 3/4 work done by crank, 1/4 work done on the crank. Intake, compression and exhaust all require force input from the crank, only power stroke drives the crank. Dec 10, 2021 at 5:47
• @PcMan ... unless you have a wide open throttle and a supercharger. Then the intake does work as well. Dec 10, 2021 at 11:12
• And as a bonus question: how many revolutions would it be for a Wankel rotary eingine? Dec 10, 2021 at 17:11

It's a little like music (if the engine is running well). Each stroke (up or down) is a half beat. So, 4 strokes is 2 beats. The power stroke is a half note, followed by three rests. In order to get 200 notes per minute, you need to play 2x the number of beats per minute.

Although the number of cylinders is not specified, if each cylinder fires at a separate time, the answer still holds, but now you have a symphony. Each cylinder must make 4 ups and downs (2 beats) to make 1 note.

Each beat requires a complete revolution of the crankshaft for that particular cylinder. With more than 1 cylinder, at the same rpm, you'll be hearing more notes per minute, which allows a larger prop, or more blades.

With a jet, it's a constant hum, but the thrust is still dependent on fuel consumption and efficiency per unit time for a given airspeed.

For 200 "fires" per minute it should look like this:

| ● ○ | | ○ ○ |

Each "measure" is turn of the crankshaft.

But for a 2 stroke engine, you would be correct:

| ● ○ | | ● ○ |

• That's quite the poetic way of putting it. I like it! Dec 9, 2021 at 16:29

On the 4 stroke cycle the cylinder fires every two turns of the crank.

For that cylinder to fire 200 times, it requires 400 turns of the crank.

That's for a single cylinder engine. If there are 4 cylinders evenly spaced, one of the 4 cylinders fires every half turn.

So for every full turn of a 4 stroke 4 cyl engine, two cylinders have fired; that is, two power pulses have occurred.

Therefore, if the crank turns 400 times in a minute, each cylinder will have fired 200 times, but if there are 4 of them, there will have been 4 x 200 = 800 power pulses in total in that minute period.

If you have a two blade prop, two blades pass by with each revolution, and there are 2 cylinder firings with each revolution. So when you spin the prop by hand, you come up on the compression stroke of one of the cylinders with each half turn of the prop, or each blade at the same clock position (props are usually installed to place the blade such that the top of a compression stroke is reached at roughly 10 o'clock blade position when standing in front facing it).

The result is the sound frequency on a 4 cyl two blade engine is same for both the propeller blade beat and engine exhaust pulse frequency (roughly 60-90 Hz from cruise to full power, and about 13 Hz at idle), which is the dominant frequency in the cockpit, and why ANR headsets, which work best at very low frequencies, are so much more effective than earplugs in airplanes.

• Good info on ANR headsets. Dec 9, 2021 at 21:36
• A few years back I put an ANR retrofit kit the Dave Clarks I bought new 45 years ago (they have the old dynamic mic), and it works pretty good, but I'd like to give a set of Lightspeeds a try. Dec 9, 2021 at 22:40
• Not sure if the number of cylinders should even be considered for a question concerning an arbitrary cylinder. The Corsair had what, 27 cylinders? Dec 10, 2021 at 2:22
• This is just relating the concept the OP asked about into a configuration that 99% of new pilots encounter first. Dec 10, 2021 at 3:37

Your misconception is about each piston being in a different stage. That's only true for balanced 4-cylinder engines.

The "Stroke" refers to the piston moving full travel up or down. You get 2 strokes per engine revolution, so 4 strokes is 2 revolutions.

It's all about scavenging, you see. Removing the burnt exhaust gases and re-filling the cylinder with fresh air and fuel. On a 2-stroke engine that happens practically as a footnote at the very bottom of the stroke.

But on a 4-stroke engine, they dedicate 2 whole strokes to scavenging. After the downward power stroke, the upward exhaust stroke (with exhaust valves opened) forcibly expels spent exhaust gases. Then comes the downward intake stroke (with intake valve open) drawing in fresh air and fuel. The next upward stroke is the compression stroke as in a 2-stroke... then the next downward stroke is the power stroke and the process repeats.

There are many advantages to using 2 whole strokes for scavenging.

• The cylinder itself is used to pump the air and exhaust, so no external "pump" is needed to force intake air in, and exhaust out. On simple 2-stroke engines, they use the backside of the cylinder as the air pump, necessitating the use of "2-cycle oil" mixed into the fuel. On large engines, they need a Roots style blower, or a turbocharger with a gear drive and overrunning clutch to forcibly drive the turbo compressor when exhaust energy is not enough.
• On a 2-stroke, the piston must be tall, so it can block the intake port in all positions except the bottom. That limits piston stroke and engine size.
• Near perfect control of gases. 2-stroke scavenging is plagued by poor performance - the engine either forces in too little air, leaving some exhaust in the cylinder; or forces in too much, pushing air-fuel straight into the exhaust pipe, where it can cause trouble. (and on an airplane, throw off your range calculations, causing unexpected stops at best!) This imperfect gas control would vary by engine RPM, throttle setting and environmental conditions (temperature, altitude, humidity).

When the engine completes each stroke, the crankshaft rotates 180 °, that is, when the engine completes a working cycle, the crankshaft rotates twice, and the flywheel and crankshaft are a whole, so the flywheel rotates twice and the camshaft rotates only once, because the number of teeth of the camshaft is twice that of the crankshaft. In a working cycle, the engine works only in the working stroke. In the other three strokes, the crankshaft is driven by the flywheel's own rotational inertia to overcome the mechanical running resistance.

4 stroke engine would mean every piston is in a different stage (intake, compression, power, exhaust)

That's only true if the total number of cylinders is four or less. If you are implying that all of these different strokes are represented at any given time, then there would have to be four cylinders, but technically, you could also say of a two-cylinder or three-cylinder engine, that with a four-stroke cycle, "every cylinder is in a different stage" at any given time.

Which means 200 power strokes every minute which would suggest 200rpm.

No, given your apparent assumption of four cylinders, 200 power strokes every minute would suggest 100 rpm. Because each cylinder fires once every two revolutions of the crankshaft, and there are four cylinders, so every half turn of the crankshaft, a cylinder fires.

Could you tell me where I went wrong?

See above. But more fundamentally, you misread the actual question ("At what speed will the crankshaft be rotating if each cylinder of a four-stroke cycle engine is to be fired 200 times a minute?") to mean that the total number of firings of all the cylinders together was 200 per minute, when the question was actually stating that any one given cylinder fires 200 times per minute. Which is why we don't need to know the total number of cylinders.