Compression Ratio - Explained

What is compression ratio? An explanation of compression ratio; what it means and why it is important. Why some cars need premium fuel, and the advantages of a diesel engine.
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[@Tain950]

I just stumbled across these video's while trying to learn more about vehicle engines and how they work. I love this kid he explains this information in such an easy way to understand. I subscribed and gave him a thumbs up.

[@EngineeringExplained]

Thanks for subscribing!

[@ArtofReliance]

+Nafir Alvarez Me too man--great job +EngineeringExplained

[@zoltancsikos5604]

Rez "this kid" Chances are that you're a 14 year old Arab kid who wants to get into steroids and sweaty men grunting at the gym.

[@clay7182]

30 is kid? 😁

[@chubitv3979]

@@ArtofReliance l

[@EngineeringExplained]

Thanks for coming back, take it in bit by bit and soon enough you'll know a ton.

[@EngineeringExplained]

Very welcome, glad you find the channel useful!

[@finnba_h]

WHAAT HE DIDNT SAY HELLOO EVERYONE AND WELCOME

[@derekbigpowers1993]

Finn H He’s come a long way

[@jpegjake]

doesn't matter, its informational

[@miguel2946]

@@jpegjake i don’t care that you broke your elbow

[@waittillthend]

9 years old

[@aaronm359]

This is the first video I've seen and I'm triggered because of this, what an unwelcome experience 😭

[@okaythen001]

googled engine compression explained and this vid came up first, awesome. keep making more good vids

[@EngineeringExplained]

It's the displacement of the engine, as you first described. Referencing this video, it'd be the difference in volume between step 1 (piston at the bottom) and step 2 (piston at the top) of all the cylinders.

[@Knowledge4yourmind]

Your videos are amazing and you explain everything from simple to complex in digestive chunks. Keep it up, and thank you.

[@EngineeringExplained]

They may use stronger pistons, if there is additional stress from the additional compression - it would be fairly application specific. You can alter the geometry of the pistons, crank, and connecting rods in order to alter the compression ratio.

[@EngineeringExplained]

Certainly, but it will take me some time. Traveling now, starting work soon, and tons of requests to fulfill! But I'll get there!

[@EngineeringExplained]

Typically 12:1 would have more power, if it were the same engine we're talking about, for multiple reasons. First, 12:1 is thermally more efficient, so less power is lost. Second, with more compression, the air has more time/room for expansion, so the power stroke is more effective.

[@EngineeringExplained]

Great to hear, that's why I made these!

[@palcolon]

Helpful, short, straight to the point.

[@EngineeringExplained]

Yep, I need to look into this more, it's pretty interesting.

[@EngineeringExplained]

Awesome, good luck to you!

[@EngineeringExplained]

Yes, it's because adding forced induction increases the internal pressure in the cylinder, so on top of that, with the compression of the stroke, the pressure would be too high, and pre-ignition could occur. By lowering the compression ratio, this can be prevented, while still maintaining high output.

[@tjj5726]

Hey Jason, fellow Mechanical Engineer here. I am doing TONS of research for this engine I am building and I want to recommend a video explaining DYNAMIC compression ratio and comparison against static compression ratio. Maybe that would shed some light to a bunch of viewers and engineers. Hope this inspires or encourages you. Thanks and I love your videos!

[@EngineeringExplained]

Refer away, I'm always looking for new material to learn from. As far as CR dictating efficiency, my source isn't wiki; that's just an easy one to check, which MOST of the time has accurate information. But if you'd like the source, it's the last book that I've referenced on the FAQ page of my website (see description) titled "Engineering Fundamentals of the Internal Combustion Engine."

[@EngineeringExplained]

Correct, to prevent pre-detonation from occurring since the air/fuel mixture will reach higher temps with higher pressures.

[@excesshorsepower1452]

Met you at SEMA, thanks for this video! I've been trying to get my head around this all day.

[@EngineeringExplained]

+ExcessHorsepower thanks for saying hello!

[@EngineeringExplained]

@1971SuperLead Correct; gasoline engines use spark plug ignition, diesel engines use the heat of compression for ignition. I explain in greater detail in my Gasoline vs. Diesel video.

[@Yathuprem]

Came here from ur new video, about VCR engines !! Totaly worth it !!

[@altius1973]

Thanks for your vids, for someone who loves cars but isn't a mechanic your explanations are easy to understand. Keep up the good work!

[@rollo6963]

I'm a 3rd year apprentice vehicle technician and my day today at collage was partly about compression ratio and that video made so much more sense than their half hour lecture 😂😂

[@EngineeringExplained]

1) Apparently yes, based on Mazda's skyactiv diesel engines. Haven't quite figured this out - may have something to do with expansion ratios. 2) The ones in Europe do, and they have higher CR as well (14:1). Direct injection and the exhaust design help them achieve this. 3) Ethanol? Well I believe ethanol has a higher octane rating (search my channel) which allows for higher CR.

[@EngineeringExplained]

Typically you'll want to use a lower CR with turbos (relative to the NA version of the engine) so that the internals can handle the extra pressure/temps and so you can prevent knock.

[@bjrn-erikmichalsen2825]

Good you wrote it. I have been searching for this reason

[@EngineeringExplained]

Correct, except for frictional losses of course. I do have a video on cylinder-deactivation as well for anyone who's interested.

[@EngineeringExplained]

If you wanted to increase the CR of your engine, yeah it could be done with different pistons, or different cylinder heads, or connecting rods, but connecting rods would likely be a complex swap. Regardless it would need to be re-tuned.

[@SavageJim01]

Very good explanation of air acting like a spring. This is the very principle that engines with cylinder deactivation use to help prevent pumping loss. They realized that if only cutting fuel supply and spark to a cylinder still requires that cylinder to do work (pump) to move air. By keeping the valves closed during deactivation, the trapped air compresses and restores to its original energy state, no net energy consumed.

[@EngineeringExplained]

Great to hear, thanks!

[@Nkprf]

I love you, you're the cheat sheet to my life.

[@agello24]

Man you have done it again!! ive learned why premium fuel is important and what "knocking" is. and the reason for compression ratios.

[@EngineeringExplained]

Change the engine geometry. New internals, domed pistons. It's not usually as simple as changing one thing, you'll have to have a group of components that work together including the crank, rods, pistons, cylinder head, and valves.

[@manuelochoa4957]

I love the way he explains compression ratio. Do you have any more engine videos?

[@bignosebilly6498]

pekis

[@EngineeringExplained]

I suppose it depends on how you increase the stroke. If the stroke increases in length (closer to the crank) and has a larger gap at TDC, then I suppose a lower compression ratio could result. Haven't read up on it or anything though.

[@EngineeringExplained]

I did indeed, thanks for the info.

[@EngineeringExplained]

Yes, increasing the stroke would be a way to increase the compression ratio, which of course alters the geometry of the crank/connecting rods.

[@robw2326]

Really excellent video. Simple and to the point without telling us a long story or screaming into the camera.

[@SERGE_Tech]

THANK YOU always finding my way back to this channel! SUBSCRIBED

[@a.c.e.7568]

You might have mentioned that when the piston is at BDC and ready to initiate the Compression Stroke, that the intake valve (and obviously, the exhaust valve) are closed.

[@teundilles]

I love it how you activelly reply to comments on your videos. Good Job!

[@Mr_T.]

Nice and simple explanation, thank you. Was wondering what the third number represents as I often see drag race tuners mention three numbers instead of two. Cheers mate:)

[@Hunter_Bauss]

Please keep making these videos!!! Me and my buddies love them. We are big gear heads and this is the easiest way to learn about engines Thanks!!!

[@lcmcnewlife11]

Thank you for a very thorough and easily-understood explanation.

[@EngineeringExplained]

It's the distance from the top of the piston (assuming it's flat) to the top of the cylinder head when the piston is at its lowest point, divided by the distance when the piston is at it's highest point. Once again, this would assume the piston and cylinder head are flat, which they are not. So really you need to measure the volumes at BDC and TDC and divide.

[@CodyDoesIt]

This is throw back as hell bit I still learned something today

[@EngineeringExplained]

No, it depends on the design geometry of the engine. But you'll typically use higher octane fuels with higher compression ratios.

[@Lisa-nz9bq]

Hey man, I have watched a bunch of your videos I really enjoy the way you simplify complex ideas. When you see 3 numbers in a ratio like; 9:0:1 what is the third number for? Also can you please elaborate on why the engine knocks because of the compression, what physically causes the engine to knock? I watched your video on knocking just not sure what makes the "ping" sound. Thank you again for your videos and explanations. Have a Great day.

[@EngineeringExplained]

@mk91077 Yes, that engine is absolutely insane. And in the states it has a CR of 13:1, with 87 octane fuel. Absolutely phenomenal engineering. Watch my video on octane ratings if you're interested.

[@awesome5me]

Wow !!! absolutely brilliant explanation...thanks for the video...you have no idea how beneficial this is for me...keep it up mate !!!

[@EngineeringExplained]

Thanks!

[@EngineeringExplained]

No need for capital letters, I respond to almost every comment. :) cc = cubic centimeters, it's just a unit of volume. A 600 cc engine is just a 0.6L (liter) engine.

[@SeanD44]

I swear, i come to your channel almost daily to watch at least two vids...some i've watched 4-5 times already...don't know a great deal about cars, just basics.

[@EngineeringExplained]

Well direct injection seems to help a lot with compression ratios. In the US the Mazda3's skyactiv engine has a 13:1 compression ratio, capable of using regular octane gasoline. In the rest of the world it has a 14:1 CR and uses premium. Not totally sure how they do it but I know direct injection plays a good role. Also, the R35 probably has a lot more boost than the 1.4 TSI, so a it's logical that a lower CR would be used. Could be wrong on that though.

[@EngineeringExplained]

Increase the CR to increase the pressure. High CR ratios are typically more efficient than lower CR engines.

[@supra392]

Man you have come SO FAR since this video. Congrats on making it man.... I love your videos they’ve helped me on occasion.

[@EngineeringExplained]

Check out my video on octane ratings. :) Altitude will not change your compression ratio, but it will change your fuel's theoretical octane rating.

[@TheBeebop420]

Very good video man! You are very good at making me visualize all of this. Thank you!

[@AB-zg5zl]

Brilliant. Clear and concise the way I greatly hope a most astute human mind could fundamentally understand engineering as a human endeavor, necessarily being capable of making the detailed knowledge practical to the layman.

[@EngineeringExplained]

Use the fuel that your car manufacturer recommends. Cooler air is more dense, yes, so everything else being equal, cooler air will have more O2. This isn't necessarily the case at higher elevations.

[@EngineeringExplained]

@IronPump89 Diesel engines have higher compression ratios because they use the heat from compression to ignite the fuel. Gas engines use a spark to ignite the fuel, so lower compression ratios are used; otherwise the fuel would ignite at an undesirable time. Yes, direct injection on gasoline exists and is now common, but a spark plug is still used to ignite the gas so lower compression ratios are still used. Though Mazda has incredibly achieved 14:1 in the Skyactiv engine for the Mazda3. Kudos.

[@samvrs]

Studying level 2 motor vehicle at college, these videos rlly help me revise 💪🏽🔥

[@TheSphongleface]

Lol when people go to school to learn something their father should have thaught them in five minutes...

[@EngineeringExplained]

Thermal efficiency, theoretically, is determined by compression ratio. Not sure what the equation is, but the theoretical thermal efficiency goes up as compression ratio goes up.

[@EngineeringExplained]

Correct, check out my video "engines - explained" for a better understanding.

[@chicagoroots4886]

Wow just happen to stumble upon an awesome channel. Thanks for making such great content

[@EngineeringExplained]

The current compression ratio of your vehicle, yes. If you haven't altered your engine at all, yes, the stock compression ratio. Should be available online or in a user manual.

[@68thelzeppelin]

Sir: You are very generous to explain these concepts for everyone to understand, I really feel grateful to you. One tiny question; How do you increase the internal pressure on the cylinder? And what does it have to do with the CR? Are Honda's engines (for ex the F20C, K20A)greater in this sense than others? Thank you in advance!!

[@EngineeringExplained]

Depends on the aspiration, but from an engineering POV the higher CR engine will typically be more efficient, so that's what I'd go with.

[@vince.r]

Watching in 2021 bro. Watched sinced the beginning. Happy for u man

[@lopezaldana7]

I like your videos dude. Keep up the good work because of you I'm learning about the stuff I love

[@anukritisaxena2107]

truly satisfying explanation. thanks so much

[@EngineeringExplained]

I have not heard much about relationships between compression ratios and turbo lag.

[@lennoxx360_yt]

had to do a presentation on this but it seems like nobody knew how to explain it to me thank you!!

[@nelsonnavarro9762]

Very good explanation, thank you very much.

[@drewmili]

You, my friend, explain like a boss

[@benjysevilla9356]

You've come along way my man

[@AdvaitThakur]

I'm always interested to use Premium fuel. After watching this video, now I know where to use such fuel. Thank you !

[@Qingeaton]

I was just trying to figure out what the average 2 cycle engine has as far as compression ratio and whether it makes sense for premix fuels to use 91 or 93 octane or not. Nice explanation of CR. Thanks

[@emanuelromo8198]

Impressive to see this video where you started

[@letsmakeithapncapn5536]

Best explanation I've heard in years

[@garethjones1827]

Nicely presented, thanks.

[@EngineeringExplained]

Mostly from a higher compression ratio. Longer stroke as well.

[@geovannygerman1939]

Finally the explanation I was looking for

[@EngineeringExplained]

Assuming the 10:1 is more efficient than the 9:1, and it's essentially the same engine, with altered compression, then yes, it could be more powerful. But you can't simply use CR to compare engines' power.

[@iamacharliest]

Thank you for posting these videos, I don't know much about cars but I am in love with jap cars myself. These video's are slowly helping me learn more about my car and I hope to be able to work on my car at a proffesional level so I can fix any problem and tinker without worry ;). I just don't know where to start :( haha but this is a good start getting to know everything first so thanks!

[@Miguel-vb4hi]

Great job explaining it. Made it real simple to understand.

[@EngineeringExplained]

Great to hear!

[@shantanukulkarni2413]

Great explanation

[@zsoltgrg]

❤❤ Watched.. Tq its helps me to understand 😊

[@thatmatty231guy]

For YEARS i never knew what this meant and i'm a damn car nut - thank you!

[@abdulwaaseihsultan7733]

Superb explanation

[@peetman23]

Hi, I have a question. The volume of compression only starts to count when the valves are fully closed right?

[@EngineeringExplained]

The engine is more efficient with high CR, so even though more power is produced, it doesn't necessarily mean more fuel is used. Higher CR means better fuel economy over a similar engine with a lower CR.

[@EngineeringExplained]

Yep, higher CR means you may have to use lower boost.

[@geraldolmstead1335]

I thank you for your many fine videos keep up the good work-Jake

[@JuanMartinez-pg3mk]

Wow it's been so long dude!

[@shakariyakollam]

nice explain thankyou

[@geoff13man]

you explained better in the first 50 seconds what it means than anyone ever has in my life 😂

[@alielmesawi9200]

Great explication mate

[@mr.robert2507]

Excellent explination. Thanks

[@959LC]

Great video again !

[@13loodLust]

You are a very excellent teacher.