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Compression increase and A/F Ratio Relationship

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Hi, I have some questions about the relationship between a compression increase, a/f ratio and cylinder temperatures.

Let's say a theoretical engine with a 10:1 Compression Ratio is tuned to perfection. You then take the same engine, and increase it's compression 1 point, to 11:1, with no other changes, and run it with the same fuel map. The first consequence of that change is that the engine will now run leaner than before, but how much? I have seen people relate a displacement increase the same as a compression increase: Say you have a 2000cc engine, and increased its displacement to 2200cc, this increase will require 10% more fuel to achieve the same A/F ratio as before at a given load (whatever it may be), considering nothing else is changed. Is that true, why?

So, this theoretical engine had a 10% increase in compression (from 10 to 11:1), is it reasonable to say it will also need 10% more fuel to achieve the same A/F Ratio?

Considering then it's running 10% leaner, and that it is running the same ignition timing as before, is it also reasonable to say it will run considerably hotter in regards to cylinder temperatures? To the point of actually seeing an increase in coolant temperatures depending on the cooling system? Is it also reasonable to say that any ignition retard applied over this same A/F ratio will make it run even hotter?


Why do you think that increasing CR would effect on how much air an engine can take? It has the same intake system, same head, same camshaft, same valves, same bore and stroke -so why the airflow would change to lean out af mixture? The amount of air trapped in cylinder mostly depends on camshaft events all else being equal and if you don'tchange your camshafts there shouldn't be any change in airflow thus af mixture. The only difference with CR change is engine efficiency (thermal and mechanical) but that has nothing to do with af mixture as a function of CR change. You might want to richen your af mixture to combat more heat in the cylinder produced by higher CR combination but again this is not function of CR change.

As georg said - I think the chap is getting confused with capacity changes and compression changes?

Thank you for the thorough explanation, Georg. As Gord said, I'm relating some concepts in a way that doesn't make sense. I was thinking that the change in volumetric efficiency provided by the raise in compression ratio, the engine would require more fuel to achieve the same A/F Ratio, which is not true, as only thermal and mechanical efficiency are affected as you said. Compression ratio changes doesn't affect how much air is trapped on the cylinder, so it cannot affect a/f ratios. As for relating displacement and compression changes, I've read this statement on a "basemap tuning guide" that's floating around. It stated you would calculate a fuel trim ratio based on the percentage increase in displacement OR increase in compression ratio to get your fuel maps in the ballpark prior to tuning.

Ah, I think I see the problem. It may be a translation issue where the "compression ratio" is supposed to refer to the absolute boost, or manifold pressure, - which makes sense. If you take normal atmospheric pressure as 1 bar, and you have 1 bar of boost, then the pressure ratio will be (1+1):1 which will need approximately 2:1 fuelling ratio.

That does make sense, Gord. But, as it is written: "[...] Engine Capacity (displacement) [referring to a tool that calculates a fuel multiplier for increased displacement] can also be helpful for calculating fuel for new compression ratios as a percentage difference: multiply your existing compression by 10 and enter it as Old Capacity. Multiply your new compression ratio by 10 and enter it as New Capacity."

Doing that for a 10 to 11:1 increase, it calculates a 1.1 multiplier, so it would add 10% more fuel to the entire fuel map, which doesn't make any sense as we discussed.

It doesn't - the fuller quote suggests either a fundamental mis-understanding or a translation error by someone with a limited understanding who was confused and tried to clear it up.

ah but if the compression ratio is higher wouldnt the peak pressure be higher, and affect the exhaust velocity, thus increasing slightly more scavenging, and a tiny increase in cylinder filling?

I don't think so - but since I have no empirical data... Anyone?

My thinking is that the higher net compression would mean the fuel is burned more quickly, imparting more energy to produce the power and so the exhaust gases will be slightly cooler (less energy remaining in it) which means it would have a lower net velocity, and so less scavenging at overlap. PVT and all that science stuff. However, been wrong before, will be wrong again ;-)

This time you are 100 percent right - higher compression ratio results in lower egt so it will slow down the scavenging - no gain in VE.

That is a good question, but I agree with Gord and Georg. With higher compression, the expansion ratio is increased during the power stroke, cooling exhaust gasses further. I don't see how that difference in pressure helps increase exhaust scavenging, in fact it does help decrease it due to the lower temperature and consequently, lower velocity as Gord said. This raises another question though, as scavenging is decreased during overlap, what would be the best way to compensate for this as scavenging greatly helps cylinder filling? You could further increase or decrease the overlap period by playing with the Lobe Separation Angle, would a longer overlap period compensate for the reduction in scavenging to help cylinder filling in this case?

On naturally aspirated engines the best way to optimize scavenging is to tune for pressure waves at specific RPM range - the difference between intake and exhaust pressure can be up to 0.7 Bar during overlap. In order to achieve that one would need to calculate for specific intake and exhaust pipes geometry that would generate positive and negative pressure waves at the same time resulting air mass moving very fast towards cylinder when the piston is still at TDC and has not even started moving down...

As Georg said. In fluid dynamic terms (been nearly 30 years, so bear with me), there are three things happening.

Static pressure head - this it the difference in pressure between the cylinder side of the valve and the port side. It's what one usually thinks of as the gas is just being 'pushed'out.

The pressure wave in the port - this is the pressure variations caused by the sudden opening of the valve and the high pressure front moving down the port and manifold until there is a section (cross section area) change or even a sharp turn - depending on the change, there will be either a positive reflection or negative reflection (basicall, the same pressure or reversed pressure) that runs back up the exhaust to the port - this may happen several times before the valve closes. By designing the exhaust system lengths correctly, it is possible to have a (or several) rpm range where there is a negative pressure wave as the exhaust closes to help draw the last bit of spent gas out and help draw the intake in, as it can be below the intake port pressure. The drawback is that if the rpm is between the low pressure points, there will be a high pressure instead, which is less favourable for exhaust gas evacuation and cylinder filling.

Velocity head - this is a property of the gas moving in the port, it's inertia (AKA momentum). As the gas is pushed out of the cylinder, it gains velocity, as the valve closes there is less gas passing around the valve (as the piston is coming to a stop) to maintain that flow and the pressure drops behind the main mass of gas moving through the exhaust, which helps draw the last of the exhaust out and fresh charge in. It is quite important the port and pipe diameter are sized to get the best velocity balance for best scavenging without being restrictive. You can get an example of this last affect simply by filling a sink with water, it takes little force to first remove the plug for it to drain, but if you try and replace the plug while it's draining there is a lot of force required to hold the plug as it's replaced.

There is plent of information on calculating the lengths, diameters, etc. If you are really keen, you could read this - - or one of the others.

Why go to all this trouble? Well, get the exhaust right, and the intake right, and you can be looking at ACTUAL volumetric efficiencies of up to 160% for a normally aspirated engine, as some F1 car, and racing motorcycle, engines have achieved!

Daniel, yes, this is a big part of why it's important to get the camshaft timing correct and a few degrees can make a big difference. Sometimes the required dimentions aren't available - pipe diameter or packaging problems - and so a small swing may help compensate for that.

Lastly, this is where the whole "tuning" thing started - way back with steam engines where they 'tuned' them for best power/torque with pipes that could be dozens of feet long!