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Variable Compression Ratio: A future technology applied today

Sunday, April 1, 2018 - 06:00
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“The engine was equipped with 12:1 pop-up forged pistons.” We who have been around awhile have all heard this statement and immediately know that this engine was built for performance. It has always been known that a higher compression ratio (CR) provides the internal combustion engine with better performance and economy. If this statement is true, why do the OEs not incorporate higher compression ratios in their engine designs? With modern improvements to the internal combustion engine in systems such as Variable Cam Timing (VCT), Variable Valve Timing (VVT), Direct Gasoline Injection (GDI), Induction Charge Valves (ICV), Forced Air Induction (FAI) (to name just a few), it is clear that the manufacturers are looking for every ounce of performance they can obtain. So why not high compression ratios? In order to understand why the OEs do not use higher compression ratios, it will be necessary to understand what is involved when changes are made to the compression within the engine. 

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Figure 1

What is compression really? 

Compression is based on a cylinder volume change that occurs over one of the strokes of the internal combustion engine. When the piston is at the Bottom Dead Center (BDC) point after the intake stroke has occurred, and then moves to the Top Dead Center (TDC) point, the volume change that occurs within the cylinder is the percent that the volume ratio changed. This volume ratio change is referred to as the static compression ratio of the engine. This static compression ratio does not change. The volume contained within the cylinder does, however.  

It is important to understand that this volume change within the cylinder of the spark ignition engine will not be constant, therefore the compression within the cylinder will be changing as well. This is because the throttle plate is varied. The throttle plate restricts the air volume into the engine, thus the air volume within the cylinder changes in correlation to the throttle plate movement. This volume change can be seen in Figure 1, which shows an in-cylinder pressure waveform using an oscilloscope and a 300PSI pressure transducer. At idle, the volume within the cylinder is low due to the throttle plate being closed, restricting the air flow into the engine. When the throttle plate is snapped open, the in-rush of air increases the volume of air within the cylinder, therefore increasing the compression.  

When the manufacturer designs the engine, they are aware of this pressure change within the cylinder. The engineer then calculates the piston movement from BDC to TDC (the swept volume) and the clearance volume that is remaining in the combustion chamber at TDC. This sets the engine’s static compression ratio based at 100 percent fill volume within the cylinder. As we now understand, this fill volume in a running engine is constantly changing so in a naturally aspirated engine, the 100 percent fill volume that sets the static compression ratio will not be reached except, possibly, at Wide Open Throttle (WOT). A passenger vehicle runs at less than 40 percent throttle opening for greater than 90 percent of the time the engine is running. Therefore, the majority of the time the engine is running, the compression of the engine is much lower than the engineer’s set point for the compression ratio. The question then would be: how is this ratio set? The data is based on the worst-case scenario that the engine could operate in, so the setting may be in Death Valley on a 125°F degree day, at WOT, with pump grade gasoline. The engine’s compression ratio is then set in these conditions so that the engine does not have detonation or overheating conditions. It would be clear that the average vehicle may never be operated in these extreme conditions but the engine design must account for possibility.  

Since the internal combustion engine is a heat engine, the fundamental operation of the device is the production and use of heat. In these engines, everything that is done prior to the combustion of the fuel type is to set up the air/fuel in the cylinder so the charge can be ignited, burned and combusted. The compression stroke of the engine takes a large volume and rapidly changes the volume state to a small volume. During these conditions the air molecules, which are comprised of approximately 79 percent nitrogen and 21 percent oxygen, hit or strike one another creating heat. The more molecule strikes that occur, the hotter the air will become. This heat is put into the working fluid, the nitrogen and the oxidant, which is the oxygen. This heat is used to heat the fuel so that it turns from a liquid to a vapor and excites the molecules so they are vibrating. These vibrating molecules will set up the charge so that it is easier to ignite and burn. In a spark ignition engine, once the point of ignition takes place the spark ionizes the spark plug electrodes producing a state of plasma which takes the fuel well past the autoignition temperature of the fuel. This sets up the ignition phase of the fuel. The combustion phase of the charge is where the chemical energy is changed to thermal energy. The heat released is then driven into the next layer of the charge thus igniting it. This is referred to as deflagration. Deflagration is the combustion that propagates at subsonic speeds through the gas that is driven by the transfer of heat. This is much different than detonation which is the supersonic shockwave that occurs throughout the combustion chamber creating a near stepwise change in pressure, this is where the charge is ignited instantly.   

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