Compression ratio, many of us have heard of this term. The common knowledge is that the higher the compression ratio, the better the performance and efficiency of the engine. So what then is compression ratio? Intuitively, it's simply the ratio of the volume of the combustion chamber (V1) when the piston is at BDC (bottom dead centre) and the volume of the combustion chamber (V2) when the piston is at TDC (top dead centre).
A more involved mathematical notation is given as follows:
where
- b = cylinder bore (diameter)
- s = piston stroke length
- Vc = volume of the combustion chamber (including head gasket). This is the minimum volume of the space into which the fuel and air is compressed, prior to ignition. Because of the complex shape of this space, it usually is measured directly rather than calculated.
Looking back at the above posted equation for the calculation of compression ratio, there are several ways you can play with the equation that will result in a higher compression ratio. Ask someone with a bit of mathematical knowledge and he will tell you what you need to change in that equation to achieve a higher compression ratio. This will then translate to the "go-fast" parts that you see available in the market.
Take Vc for example, by decreasing Vc, the resultant value of the CR will automatically increase, this translate to parts like high-comp head gaskets, which in fact are gaskets that are thinner than stock, that reduces the volume Vc. High-comp pistons do the same thing by introducing pistons with thicker/higher crowns which in turns reduces Vc.
Another way of increasing the compression ratio is to add a stroker crank. And you also order new pistons with the pins mounted farther up so that the piston does not intrude farther into the combustion chamber at TDC. Thus your combustion chamber volume is unchanged - all that changes is the stroke. You will still increase your compression ratio, as the equation points out.
Nominal vs Dynamic Compression Ratio
After reading the above section, you may be tempted to conclude that you have acquired all there is to know about compression ratio, but wait, there's more. All the above calculation and example only points out the compression ratio of a static engine thus the name, nominal compression ratio or static compression ratio. This is something that is calculated on paper and is only as accurate as the met station forecasts. Now to add in more moving parts into the equation.
Valves and camshaft timings, these are directly responsible for screwing up your nice little equation you had earlier on achieving your expected compression ratio. It may seem intuitive that higher compression ratio will mean higher cylinder pressure when the piston reaches TDC. However the cylinder pressure (prior to ignition) is dependent on what is loosely referenced as "dynamic compression ratio".
Valve opening and closing timing and duration greatly affects this "dynamic compression ratio". Specifically the intake valve closing point directly affects the dynamic compression ratio.
During the compression stroke of a performance engine operating at high RPM. Most are probably familiar with the strokes in a 4-stroke engine:(1) intake (2) compression (3) ignition (4) exhaust.
Obviously during the intake stroke the intake valve must be open in order to let the air/fuel mixture into the cylinder. Then at BDC the intake valve closes so that the piston can move up and compress the mixture right? Wrong! The intake valve on a modern performance engine stays open well into the compression stroke.
Notice above that the piston has moved past bottom-dead-center (BDC), and is on its way up the bore in an attempt to compress the air/fuel mixture prior to ignition. Yet the intake valve is still open. In fact, with a any kind of performance cam the intake valve will not close until 50° - 75° past BDC! That's 28% - 42% of the way into the compression stroke!
Static compression ratio is directly related to stroke. In principle the piston cannot compress the mixture until the intake valve closes. Thus if the intake valve closes when the piston has already moved quite some distance up the bore, then the amount that the intake charge will be compressed is reduced. The "effective compression stroke" has been reduced. Does this mean that when an engine is operating that the dynamic compression ratio is lower than the static compression ratio? Well yes and no.
An engine with a performance cam operating at low RPM will suffer a loss of torque due to the fact that the effective compression ratio is reduced by the late intake valve closing point. However, as the RPM increases "inertia supercharging" becomes important. At high RPM's the intake charge is is moving into the cylinder at high velocity. As such it has a lot of inertia and will continue moving into the cylinder past BDC, even though the piston has changed direction and is now moving up the bore (towards the incoming charge). Ideally the intake valve will close just before the incoming air stops and reverses direction. This guarantees that the maximum amount of air/fuel mixture has been drawn into the cylinder prior to ignition. When this happens an engine is said to have "come on the cam". In order to ensure that the mixture is still compressed sufficiently over the reduced effective compression stroke it is necessary to increase the static compression ratio. This is why high performance engines with aggressive camshafts also tend to have high static compression ratios.
A mild cam with an early intake valve closing point will work well at low RPM. But at high RPM the intake valve will close before the maximum amount of air/fuel mixture has been drawn into the cylinder. As a result performance at high RPM will suffer. If a high static compression ratio is used with a mild cam (i.e. and early intake valve closing point) then the mixture may end up being "over-compressed". This will lead to excessive compression losses, detonation and could even lead to head gasket or piston failure.
On the other hand, an aggressive cam with a late intake valve closing point will work well at high RPM. But at low RPM the intake valve will close too late for sufficient compression of the intake charge to occur. As a result torque and performance will suffer. If a low static compression ratio is used with an aggressive cam (i.e. a late intake valve closing point) then the mixture may end up being "under-compressed". Thus a high performance cam with long duration should ideally be combined with a higher static compression ratio. That way the engine can benefit at high RPM from the maximized amount of intake charge afforded by the late intake valve closing, and still achieve sufficient compression of the mixture as a by-product of the dynamic compression ratio.
This is evidently in the different kinds of performance camshafts available in the market segmented for the people with different usage patterns. For example Yoshimura produces for the Suzuki GSXR 1000, 3 different types of camshafts with different lift and durations, namely Stage1 for street use, Stage R Type S for slightly more involved sports riding and finally Stage R Type R for race applications. It'll be intuitive to know how badly the low end will suffer with very high lift cams. Thus leading to the mention 3 stages of segmentation.
Below shows the different camshafts offered by Yoshimura.| MODEL | YEAR | PART# | STAGE | LIFT | DURATION | |||
|---|---|---|---|---|---|---|---|---|
| | SUZUKI GSX1300R GSX-R1000 GSX-R1000 GSX-R1000 GSX-R1000 GSX-R1000 GSX-R750 GSX-R750 GSX-R750 GSX-R750 GSX-R750 GSX-R600 GSX-R600 GSX-R600 HONDA VTR1000 YAMAHA YZF-R1 | ------ 99-06 05-06 05-06 01-04 01-04 01-04 2006 04-05 01-03 98-99 96-97 2006 04-05 01-03 ------ 97-99 ------ 98-01 | ---------------- SUXR341 210-506-0100 210-506-0200 210-503-0100 210-503-0200 SUXR481 210-568-0200 210-588-0100 SUXR471 SUXR241 SUXR141 210-567-0200 210-567-0200 SUXR461 ---------------- SUXR321 ---------------- SUXR441 | ---------- STAGE-1 TYPE-R TYPE-S TYPE-R TYPE-S STAGE-1 TYPE-S STAGE-R STAGE-1 STAGE-1 STAGE-R TYPE-S STAGE-R STAGE-R ---------- STAGE-1 ---------- STAGE-R | ----------- 9.00/7.80 10.00/8.60 9.40/8.10 10.00/8.30 8.72/7.72 9.50/8.20 TBA 9.20/7.60 9.20/7.60 9.00/7.80 9.00/7.80 TBA 8.40/7.00 8.40/7.00 ----------- 10.70/10.70 ----------- 8.20/8.60 | ---------- 244/230 256/252 260/254 253/240 252/245 248/230 TBA 250/228 250/228 244/230 244/230 TBA 260/234 246/231 ---------- 250/250 ---------- 250/248 | |
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