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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[Tain]

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 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!

[rollo]

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 😂😂

[SavageJim]

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.

[finnba_h]

WHAAT HE DIDNT SAY HELLOO EVERYONE AND WELCOME

[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.

[Knowledgeyourmind]

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

[okaythen]

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

[palcolon]

Helpful, short, straight to the point.

[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.

[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.

[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.

[EngineeringExplained]

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

[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.

[tjj]

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]

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.

[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]

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]

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."