You’ve heard the term compression ratio before, but have you ever wondered exactly what it means? Well, it’s time to explain exactly what compression ratio is, and why every carmaker is now obsessed with it like it was the Holy Grail.
Compression ratio, admittedly, is more complicated than it seems at first. It doesn’t help that it’s one of those terms you hear getting thrown around at car meets and in press releases without much serious explanation. It’s one of those things that you mostly pretend to understand while trying to impress that trapeze artist you met at the circus last weekend.
We know that high compression is good, and that low compression is bad. We know that Mazda’s new Skyactiv-X “Holy Grail” engine is high compression, along with Infiniti’s “diesel slayer” and Toyota’s “Dynamic Force” series that all advertise more power along with more efficiency.
We live in an age where engineers can’t just give an engine more power by making it bigger. Changing an engine’s compression ratio is becoming how it’s done.
(By the way, if you’re reading this and snorting because you already know what compression ratio is, good for you! Not everyone else does.)
What Defines Compression Ratio Is Super Simple
A compression ratio is exactly what it sounds like—a ratio where you’re compressing the maximum cylinder volume into the minimum cylinder volume. That’s the volume of the cylinder when a piston is all the way down compared to all the way up to the top. It’s written out and said as a ratio. For example, for an engine with a 9:1 compression ratio, you’d say that it’s “nine to one.”
Now picture a cylinder in your head. The piston moves up and down inside that cylinder. When the piston is at the lowest point, that’s called Bottom Dead Center. That’s where the cylinder volume is greatest. When the piston is at the highest point within the cylinder, that’s called Top Dead Center, and that’s where cylinder volume is smallest. The comparison of these two volumes is where your ratio comes from.
If you are a visual learner like I am, you’ll like this GIF I made showing how a four-stroke engine works. See how the piston moves up during that compression stroke? That’s all the air and fuel getting compressed in the cylinder. If an engine has a high compression ratio, it means that a given volume of air and fuel in the cylinder is being squeezed into a much smaller space than an engine with a lower compression ratio.
And now for an example with simple math, my favorite kind.
Pretend you have an engine whose cylinder and combustion chamber volume is 10 cc when the piston is at bottom dead center. After the intake valve closes and the the piston rises up during the compression stroke, it squeezes the air and fuel mixture into the space of a single cubic centimeter. This engine has a compression ratio of 10:1.
That’s it! That’s compression ratio. Total swept volume plus compressed volume (including the volume of the cylinder head and everything above where the piston “sweeps”) into compressed volume alone.
Why It’s Better Is Complicated
But understanding what compression ratio is is less important than understanding why we care about it, or why high compression is such an aspiration.
The best explanation I got in this came from my coworker and engineer David Tracy, who then reached out to other engineers and professors for help. The best answer of those came from Dr. Andy Randolph, Technical Director at ECR Engines. He does powertrain research for NASCAR, and his explanation is super clear:
From a laymen’s perspective, engine power is generated when combustion exerts a force on the piston and pushes the piston down the cylinder during the expansion stroke.
The higher the piston is in the bore when combustion begins, the more force will be exerted.
As compression ratio increases, the piston moves higher in the bore at top dead center, hence there is additional force for the expansion stroke (additional force for the same amount of fuel equals higher efficiency).
Now, we really should understand more about the why in addition to the how, and that means we’re going to have to venture into the realm of thermodynamics.
The basic point of this all is that a higher compression ratio means that the engine is getting more work out of the same amount of fuel. That’s good for power and also miles per gallon.
To explain why a higher compression ratio yields better efficiency, we’re not going to dive too deep into thermodynamics, but what the hell, let’s just dip our tippy toes in. It’s healthy and good for the soul.
Higher Compression Means More Work But More Pressure
The image above shows a P-V, or Pressure-Volume, diagram for an ideal and typical gasoline engine. It shows, visually, what’s going on in your engine as it burns gasoline.
In the diagram above, that lower 1-2 curve shows the compression stroke.
The 2-3 line shows combustion.
The upper 3-4 curve shows the expansion stroke.
And the 4-1 line shows heat rejection when the exhaust valve opens up.
To be more technical, in the diagram the 1-2 curve shows the compression stroke in which pressure (y-axis) rises and volume (x-axis) drops as the piston does work on the gas, squeezing it. The 2-3 line shows the heat released during combustion, quickly increasing the pressure and temperature of the gas. The 3-4 curve shows the volume rising and pressure dropping as the gas does work on the piston during the expansion stroke. The 4-1 line shows heat rejection from the gas to the surroundings as the pressure returns to ambient with the exhaust valve opening up. Finally the flat 1-5 line on the bottom represents the exhaust stroke and piston returning to top dead center at the end.
The area within those 1-2-3-4 lines represents how much work done by the engine. Higher compression ratio means the two vertical lines on the plot will move to the left and up, leaving more area within the bounds than with a lower compression ratio, and thus work being done. But as you can see on that diagram, you would hit a higher pressure. Put another way, you’d wind up with more mechanical work from your high-compression ratio engine. You’d be getting more pressure in the cylinder and on your piston from the heat input from combustion.
Higher Compression Also Means More Thermal Efficiency
It’s also important to note that the heat input and heat loss during the cycle of your engine relates to the efficiency as a function of compression ratio. The whole thing works on two ideas. The first is that whatever heat energy enters the system must be converted to either mechanical work or waste heat. The second is that thermal efficiency is simply the work output divided by the heat input. So then you can derive the relationship between thermal efficiency and compression ratio, like MIT plotted on its web page and shown above. The equation is here (nu is thermal efficiency, r is compression ratio, and gamma is a property of the fluid):
As you give an engine of a certain displacement a higher compression ratio, you effectively shift the P-V diagram up and to the left, and increase heat input (Qh in the diagram) more so than heat loss (Ql). Put another way, you’re turning more of your input energy into work. Here’s Jason Fenske of Engineering Explained breaking down that relationship between compression ratio, heat transfer and efficiency:
Anyway, the point is that thermodynamics dictates that thermal efficiency goes up with compression ratio, as you can see by that plot and equation above. And that means more horsepower, better fuel economy, heavier wallets and bigger smiles. Drive any sluggish, wheezing, gas-sucking, old low-compression American V8 and you’ll know what I mean.
Compression ratio is also what makes engines like Mazda’s Skyactiv-G engine so efficient. The first of a wave of new high-compression and variable-compression engines from Mazda, Nissan/Infiniti and Toyota, the Mazda has the highest compression ratio in the business right now, at 14:1, which is why it can manage high fuel economy and power figures even without a turbocharger.
Why Higher Compression Means You Need Higher Octane
Why doesn’t everyone just use high compression ratios? Well, high compression is why a lot of performance engines need premium fuel, or high-octane gasoline. Octane ratings are, as this How Stuff Works article points out, a measurement of the gasoline’s ability to resist detonation.
Compared to gas with a high octane rating, gasoline that has a low octane rating is more likely to auto-ignite due to high air-charge temperatures and pressures. Basically, you want the gas that ignites when you want it to, not the kind that ignites when you don’t want it to. That kind of uncontrolled combustion is called knocking. Knocking is bad; it reduces torque and can cause irreparable damage to your engine.
High compression increases your risk of knocking, which is why very high compression engines run high octane race gas or (more commonly now) E85. Gases tend to heat up when they’re compressed, so the increased heat density could lead to the fuel prematurely combusting before the spark plug ignites it. To reiterate: That’s bad.
Mazda had to do a lot of work to its piston and exhaust design to mitigate knocking on its 14:1 engine running on pump gas. The pistons in a Skyactiv-X engine, for instance, have a cavity in the middle, to allow for Mazda to shoot a burst of rich fuel around the igniting spark plug in an otherwise lean mixture and, yeah, there’s a reason why this wasn’t an easy technology to develop.
What’s also interesting is that you can’t just make an engine with as high a compression ratio as you want. I reached out to John Hoyenga, an owner at the performance exhaust and rally shop Nameless Performance, to chat about risks and benefits of high compression.
John is building a Nissan 240SX rally car into which he’s swapping a SR20VE four-cylinder, currently making about 250 horsepower at the wheels from just 2.0 liters. This is, surprisingly, with no turbo. All John has to thank is its very high 14.5:1 compression ratio. “There’s more work done by compression,” he explained, “so the more power [an engine] will make without boost.”
That being said, because this is a race engine, he’s running it with race gas or extremely high octane E85. John said that anything over 14.5:1 compression ratio would run the risk of auto-ignition, and it could shoot out a rod or spin a bearing. This is what’s casually referred to as “blowing up.”
There’s A Limit To How High You Can Go
I asked if this is why we don’t see people aren’t running around with engines that have significantly higher compression ratios than anything we see today. Obscenely high ratios, like 60:1. John laughed. He explained that metal simply cannot withstand such high levels of stress, and a compression ratio like that would run things so hot that it’d blow up any current engine.
Of course, not all of us are building race cars with race engines, so altering compression ratios isn’t something we ever have to worry about. But we are casual car owners and quasi-engine enthusiasts, so this was an explanation for what compression ratio means and why it matters. You don’t have to fake it anymore, you now know what it is.
Now, go and find that trapeze artist and tell him how you feel!