Why dynamic compression ratio is nearly useless

For years people have relied on the term dynamic compression ratio to explain somethign much more complex than it can explain.

 

More on DCR can be found here.

http://en.wikipedia.org/wiki/Compression_ratio#Dynamic_compression_ratio

 

 

In short though DCR measures the volume of the the cylinder when all valves have closed (which happens when the piston is part way up the cylinder) and compares it against the volume in the combustion chamber at TDC.

 

There is a huge problem with this. For starters this is not a dynamic measurement so the name is very misleading. Dynamic means in motion. DCR is measured at a static point.

The theory is that as the piston pushes up it will push air back out the intake and or exhaust valves until they close. Therefore the remaining air in the cylinder will contain that volume and mass of air at atmospheric pressure. This only works at zero RPM. As RPM increases you have pressure waves and delayed response in pressure and direction of travel. As the piston speeds up the mass of air that stays in the cylinder changes. This is called volumetric efficency.
You may have noticed that I said mass but the term is volumetric. The volume of the cylinder is constantly changing so inside the motor you can't think in volume. You have to give that volume a fixed pressure then volume will be the same as mass.
Volumetric flow rate (VFR) and volumetric efficiency (VE) are generally calculated at atmospheric pressure. So if the motor is running at 80% VE then that means that the air going into the air filter at atmospheric pressure will be 80% the swept volume of the cylinder each time it takes in a new charge.

You can learn more about VE here.
http://en.wikipedia.org/wiki/Volumetric_efficiency

 

Let's go into a little more detail on actual dynamic cylinder filling.

 

I will start at the end of the exhaust stroke because it is important to the beginning of the intake stroke.

As the piston travels up the exhaust valve opens and the piston starts pushing the exhaust out. The motion of the air flowing out the exhaust creates a low pressure zone in the combustion chamber so while the exhaust valve is still open the intake valve opens. The air moving out the exhaust will start pulling air into the cylinder. If it's timed right the exhaust valve will close just as all the exhaust is expelled and before any fresh air and fuel gets pulled into the exhaust. 
 

As the piston goes down it creates a low pressure zone because the air cannot fill the space as fast as the piston is dropping. When the piston hits the bottom of the stroke the pressure in the cylinder is much lower than atmospheric. Because of this, air is still rushing in through the intake valves as the piston starts back up the cylinder. As the piston starts to travel back up the cylinder it starts to create a high pressure wave at the bottom pushing the air back up while low pressure is still drawing air in at the top of the cyl.

The goal is to close the intake valve right as the pressure equalizes at the valve throat. As much air has rushed in as can rush in and the valve closes before any can rush out. At this point the motor has as much air in the cylinder as is possible at that RPM.

The thing to remember is that the faster the motor spins the longer it takes the air pressure to equalize. At higher RPM the piston will be further up the stroke when the air equalizes. The higher up in the RPM the earlier you want the intake valve to open and the later you want it to close. This is what is called duration and why larger duration cams make more power at higher RPM.

 

So now we can see that the duration of the cam effects how much air the motor can take in at a given RPM. It should also be quite clear that DCR is only useful or accurate at zero RPM and that the faster the motor spins the less DCR tells you anything about anything.


What does tell you something is volumetric efficiency.

VE measures how much air moves through the motor per revolution in relation to it's swept volume.

From VE we can calculate effective compression ratio. Now this is a very useful number. This tells you what your actual compression ratio will be at that moment. Let's say we have a 2 liter motor with stock cams. Since each cylinder only has an intake stroke every other revolution this motor at 100% VE will move one liter per revolution.

Now if the intake valve closes when the piston is 20% of it's way up the cylinder then at very close to zero RPM the motor will have 80% VE. Here DCR, VE and effective compression ratio line up.
 

As the motor starts spinning faster and as it starts getting closer to the RPM it was designed to be optimal in the VE should actually start to increase. Let's say at 4000 RPM this motor is consuming .9 liters of air. Now it's consuming 10% more air than the dynamic compression ratio.

Let's say by 6500 it's reached it's VE peak of 100% and is now consuming 1 liters of air. Now it should be apparent that DCR really means nothing to a running motor. It doesn't tell you anything about how that motor will behave or what it's effective compression ratio will be. At this RPM your motors effective compression is the same as the static compression ratio and the dynamic compression ratio is completely irreievant. 

Let's take another motor. Same motor but race prepped. Now let's say the intake cam on this motor closes when the piston is half way up the cylinder. This will give you a DCR of half the SCR. In other words at very close to zero RPM half the mass and volume will stay in the cylinder. The other half will have been pushed back out the intake and possibly exhaust. The motor is now consuming .5 liters of air per revolution. Pre ignition pressure will be much lower, combustion pressures will be much lower. Internal stresses will be much lower and power output will be much lower. Lower cylinder pressures also mean it is further away from detonation in this area of the power band. This means you could run much higher compression and ignition timing in this area without getting detonation.

As the RPMs climb the motor will become more efficient. Let's say on this motor by 6000 RPM the VE is up to 80%, this is much different than the 50% DCR.

Let's say that it hit it's peak VE of 110% at 8000 RPM.
At close to zero RPM the motor would have injested .5 liters of air per revolution. Now at 8000 RPM the motor is injesting 1.1 liters of air per revolution. The cylinders pressures before combustion are much higher. One key thing about higher RPM is that the piston is traveling away from the head faster. This means that at higher RPM combustion pressures will stay lower. Everything also happens much faster. These two things make it much harder to detonate so you can have higher cylinder pressures and burn more air and fuel without concern for detonation.

 

So now if you have made it this far you will see the concept is very complex and there is a lot going on.
DCR is almost completely useless but it is the oversimplified way of explaining something most people wouldn't be able to or wouldn't care to take the time to understand fully.

You could run two completely different cams and as long as the intake valve closed at the same time they would have the same DCR yet power output would be completely different, where the motor got detonation would be completely different, the VE curve would be completely different but you wouldn't know by looking at DCR.

 

Effective compression rati tells you much more. It's not as easy or easily available to properly calculate unless you actually have some form of VE curve tables but even without them you can tell quite a lot about cause and effect. Going from a 250 cam to a 276 cam will move the VE curve up in the power band. With the same compression the motor will make less power in the lower RPM because less air in entering the motor per rev. At higher RPM the motor will ingest more air and therefore make more power.

Since detonation is much more likely in lower RPM the reduction in filling in that area would allow you to run more compression. This will help you recoup a lot of the power lost from the reduced VE in the lower RPM. A high performance motor with big cams and properly matched compression should be able to make similar power numbers in the low to mid RPM as it's stock or mildly tuned counterpart while making much more power in the upper RPM.

If you have access to VE maps or if you have aftermarket engine management where you can calculate VE based off of say AFRs and injector duty cycle then you can do a lot more. To calculate actual effecive CR you would just take the volume/time devided by revolutions/time to get volume per revolution.

On a four stroke you will then divide the displacement by 2 because it only has an intake stroke every other rotation. Now you just divide the volume per rev by the halved displacement of the motor.

For example

If the motor consumes 2900 LPM at 4000 RPM 2900/4000 = .725 liters per rev.

The static volume/rev on our 2 liter motor example is 1 liter. 

.725/1 = .725 or 72.5% VE.

On a 1.6 liter motor with .8 l/rev the VE would be 90.06%

 

 

CR: Compression Ratio

SCR: Static Compression Ratio

DCR: Dynamic compression rati

VE: Volumetric Efficiency

 

courtesy of webmatter.de