The Only Video You'll Ever Need to Watch to Know how 4 Stroke and 2 Stroke Engines Work and Differ
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Published 2022-12-04
Let's start with the four stroke. Why is it called a four stroke? It's because the four stroke engine needs four strokes to complete one combustion cycle. Every time the piston moves from top to bottom............. or vice versa............... that's one stroke. One stroke of the piston equals 180 degrees of crankshaft rotation.
Now the four strokes are: intake... compression..... combustion..... and exhaust
During the intake stroke the piston travels from top to bottom, which are also known as top dead center and bottom dead center. As the piston does this it creates an empty space or vacuum inside the cylinder. This newly created void is essentially a brief absensce of air, and because we have an abscence of air we also have an abscence of air pressure. In other words we have low air pressure inside the cylinder and atmospheric air pressure outside the cylinder. This air pressure difference cannot continue to exist and air naturally seeks to equalize pressure everywhere, and so air together with fuel from the outside rushes into the cylinder and fills it with a fresh air-fuel mixture.
Although cobmustion inside an engine is often described as a bang or explosion, that isn't what's actually happening. An explosion is detonation, which is a rapid uncontrolled process. In contrast to thist, combustion is deflagration, which is a much slower, more even and controlled process.
Combustion spreads out evently outward from the spark plug though heat transfer. The small portion of air fuel mixture initially ingited by the spark plug heats up and ignites the next layer of the air fuel mixture until all of the air fuel mixture is burned.
As the combustion flame front spreads it rapidly raises the tempreature and pressure inside the cylinder. Because the cylinder is sealed this pressure has nowhere else to go so it ends up pushing the piston down the cylinder with great strength....... This is our combustion stroke and of the four strokes this is the only one that actually generates power and it does so by converting the energy released by combustion into motion of the piston which then turns the crankshaft and ultimately the wheels.
By the time the piston reaches bottom dead center again all the air fuel mixture has been burned and we now have ehxuast gas, or the remains of combustion inside the cylinder.
Now in the case of the two stroke we have a cylinder head together with the valve cover whereas in the four stroke we really have just a cylinder cover or cap. There are zero moving parts in the 2 stroke head. No valves, no chains, no cams, no springs and therefore less weight, less complexity, less cost and less potential for failure.
Each time the 2 stroke piston is at top dead center a combustion event occurs. This also explains the name „two stroke“. The engine only needs two piston strokes to complete its combustion cycle. In other words each 360 degrees or one full engine revolution results in a combustion event. Whereas in a four stroke combustion occurs only every other time the piston reaches top dead center, combustion occurs only every 720 degrees of engine rotation. This means that the two stroke produces twice as many combustion events or power pulses for the same rpm, which means that, at least in theory, the two stroke engine can make twice as much power from the same displacement compared to a four stroke.
The other important thing we can note is that the strokes are very clearly defined and separated in a four stroke. Each completed stroke of the piston marks the beggining of one and the end of another stroke of the combustion cycle. But the two stroke sort of lumps the strokes together, they overlap and occur simultanously. The two stroke is actually multi-tasking which enables it to squeeze more action into the same time-frame. But as with everything, there is a price to be paid.
A special thank you to my patrons:
Daniel
Pepe
Brian Alvarez
Peter Della Flora
Dave Westwood
Joe C
Zwoa Meda Beda
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Cole Philips
#d4a #2stroke #4stroke
00:00 4 stroke combustion cycle
08:02 2 stroke combustion cycle
13:24 Reed valve
15:02 Lubrication
21:12 Compression ratio
23:41 VVT & Power valves
27:18 Direct Injection
All Comments (21)
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i am a university professor in mechanical engineering and I have never seen a more practical way of explaining the difference between 4 stroke and 2 stroke than this video without sacrificing the crucial details. Many thanks! I will also share that with my students.
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As a retired engineer /professor; I can surely say that this gentleman gives beautiful explanations of rather complex issues. Well done!
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The 4 stroke always made more sense to me… the 2 stroke has always been more a mystery. I’ve done some rebuilding and porting on 2s, but damn whoever came up with that idea is pretty clever imo. The 4 stroke is very straightforward technical engineering and the 2 is extremely clever outside the box engineering.
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Nothing on YouTube comes close to this video it's amazing
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I can’t even quantify how much I’ve learned from you over the last few years 🤯
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I don't know what motivates you to make these videos, but I hope you never lose it. As a fellow gearhead, it's sometimes challenging to explain to a non-gearhead why things are designed the way they are, but you do it so simply and eloquent, I've often referred others to your videos instead of trying to explain it myself. In fact, I watch your videos with my 8 and 9 year old son's all the time so they understand better how things are built. Your videos are invaluable and I sincerely appreciate your effort
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As a professional stay at home astronaut, huge props for communicating such complex topics with such amazing clarity
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The best 2 stroke video I've ever seen. I was a powersports dealer selling & working on 2 stroke snowmobiles for a decade and I learned more in the last 10 minutes than I did in 10 years working on them. Thank you!
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That was incredible! As an experienced Mechanical Engineer, I know these 2 systems pretty well and understand them, but you re probably the only guy who can explain it so so well so easily to EVERYone. Awesome job!
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50 years a mechanic, but thoroughly enjoyed this. Beautifully done. Just shared it with my son who is an apprentice mechanic. Absolutely marvellous.
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I am not a professor, but a usual housewife with no knowledge of mechanics. But I must say, I understood this stuff quite easy thanks to great pictures, models and that magnificent voice!
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Respect for this guy. His explanations about mechanics are always precise, entertaining and easy to understand. Best YT in its class.
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I'm a retired auto mechanic, and I've never completely understood how a two-stroke works. Your presentation was, quite simply, amazing! Thank you!
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I have been a motorcyclist for 53yrs. Built tons of bike, both 4T and 2T (the latter my favorite). I appreciate BOTH segments, and have owned multiple (30+bikes). This was THE best video that I have ever seen to help me explain to others the differences between the engine designs (and honestly helped me) ! Wonderful video, you did a great job. Your filming, production, explanation where all world class.
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I am also a Professor like some of your other commentators (medical, not mechanical engineering), and I too want to compliment you for what is easily the BEST explanation of a 4 stroke vs. 2 stroke engine! You have a true gift for breaking down complex concepts! Thank you!
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I remember 40 years ago my grandfather explaining to me the difference between 2 stroke and 4 stroke engines. From his explanation I had a rudimentary understanding of 2 stroke engines. Your video greatly deepened my understanding of 2 strokes on top of the knowledge he gave me. Thank you!
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I've been teaching my son since he could walk about cars, motorcycles and all things mechanical. He's 24 now. He has commented back that your videos have opened the door into this world. Like me, he's a visual learner. I can learn more from a 20 min video that a 3 hr lecture. Your illustrations and visual aids at this point are one of the best that I have come across on YT. Great vid. Keep it up!
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If you aren’t a teacher, you definitely should be. I have watched quite a few of your videos and the way you explain everything in detail the way you do is amazing. Keep up the good work my friend. Thank you for the knowledge 🙏
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Cannot believe that this is free content. This is probably the best comparison of four stroke to two stroke engine ever made
