“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.
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.
Once the fuel reacts with the oxidant, the thermal energy released heats the working fluid which causes the nitrogen to expand and push down on the piston surface area. This in turn uses the three-bar linkage to produce torque from the crankshaft. So, chemical energy is turned into heat energy which is turned into mechanical energy.
Increasing efficiency
Compression is the volume change that occurs within the cylinder; the higher the compression the greater the heat that is put into the cylinder. Since the internal combustion engine is a heat engine this additional heat will create more output from the engine. Let us be clear here, the air/fuel charge only burns at one rate producing one value. The additional power does not come from the fuel burn but rather the additional pressure that is produced from a higher compression within the cylinder. With the piston coming physically closer to the head there is less area which will produce a higher peak pressure. This higher peak pressure will increase the engine’s thermodynamic efficiency which is a measure of how effectively the engine converts heat into mechanical power. In Figure 2 a chart is shown that demonstrates the theoretic thermodynamic efficiency gains as a product of compression ratio.
To understand how this occurs it is necessary to look at the engine’s expansion ratio. The expansion ratio explains what occurs as the piston is moving downward while the fuel is burning, creating pressure within the combustion chamber. Since the piston came physically closer to the head there is less area within the combustion chamber. As the fuel releases its thermal energy it heats the working fluid which creates pressure within the combustion chamber. Pressure is the force multiplied by the area. Pressure per Square Inch (PSI) or, more accurately, pound-force per square inch, is the force of one pound-force applied to an area of one square inch. So the pressure within the combustion chamber is multiplied by the area of the piston. Thus the higher the pressure the more force is created to push down on the piston. The rule of thumb for a gasoline based engine is that the compression ratio is about a hundred times the combustion pressure. So a CR of 8:1 would produce approximately 800 PSI of peak combustion pressure, whereas a higher CR of 12:1 would produce approximately 1200 PSI of peak combustion pressure. For example, if a 3 inch diameter piston were used; 3 inches / 2 = 1.5 radius, 1.5 radius x 1.5 radius = 2.25 radius squared, 2.25 radius squared x 3.14 pi = 7.065 area of a 3 inch piston. Now that we have the area of the piston multiply this by the force, 7.065 x 800 PSI = 5652 pounds of peak force, and 7.065 x 1200 PSI = 8478 pounds of peak force. It is now clear that the compression ratio produces a higher force to rotate the crankshaft with, thus producing greater performance.
Additionally with a higher compression ratio the volume ratio change within the combustion chamber has a greater change over the power stroke as well. With a higher compression ratio the area at TDC is smaller so the area, or volume, will have a greater change as the piston moves away from the head. This area will change the way the peak pressure in the cylinder decays. An increased area allows the burning fuel to expand with a greater force over more degrees of crankshafts rotation, thus more energy is extracted from the original high-pressure charge. This in turn helps the engine’s thermal efficiency.
Higher compression ratios produce higher peak pressures, therefore the design of the engine components will be heavier in order to withstand this greater force. However, there will be a penalty for carrying this addition weight for the life of the vehicle, so the gain from a heavier engine must be offset by better performance produced from the higher compression ratio. Additionally, there will be a limit to how high the compression can go. The physical properties of the materials used in the engine as well as the fuel stock will have limits. Ultimately the engine cannot have detonation, as detonation will create severe engine damage, so the compression ratio must be set to eliminate detonation within the combustion chamber.
The answer — VCR
Now that the problems of setting the static compression ratio in an engine are obvious, what is needed is a way to alter the compression ratio of the engine. We are all aware of one such static compression ratio change in the engine, known as the cold start enrichment. When the additional fuel is added to the combustion chamber on a cold engine, the fuel stays in a liquid format. Liquid, being virtually incompressible, takes up some of the clearance volume in the combustion chamber. This lowers the clearance volume within the combustion chamber thus increasing the compression ratio of the engine. The additional pressure that is produced from a higher compression ratio increases the temperature of the working fluid, thus with more heat the lighter aromatics of the fuel stock flash into a vapor. Remember only a vapor can burn; liquids and solids do not burn. Additionally, only an air/fuel mixture being stoichiometric can burn. If the air/fuel mixture is rich, once the oxygen is consumed the fuel will no longer burn leaving fuel in the combustion chamber; and if the air/fuel mixture is lean, once the fuel is consumed there will be oxygen left in the combustion chamber. Once enough of the cold start enrichment fuel vaporizes, then the air/fuel mixture is combustible and the engine can be started. This is a temporary compression ratio change. What is needed is a way to accomplish this clearance volume change on a permanent basis.
The best way in which to utilize the engine’s compression ratio is to dynamically change the compression ratio as the engine is running. The Variable Compression Ratio (VCR) engine does just that. The VCR engine changes the volume within the cylinder so that the compression is changed on the fly. There are many ways in which this can be accomplished, however in the few VCR system examples shown in Figures 3-7, the change of the clearance volume at TDC is how this will be accomplished. When the compression ratio can be changed dynamically, the best compression ratio for the conditions that the engine is operating under can be utilized. This means that under light load the static compression can be much higher than the static compression under heavy load. This increase of the static compression ratio under light engine load conditions increases the thermodynamic efficiency of the engine.
Under light load the cylinder fill volume is far less than 100 percent. This is due to the throttle plate and the air flow velocity moving through the engine. With less volume fill within the cylinder the compression pressure is much lower than the static compression set point. So if the static compression ratio is raised, with less volume contained in the cylinder, the pressure within the cylinder goes up creating high fuel efficiency. Under heavy engine load where the throttle plate is at WOT the volume contained within the cylinder is high so the static compression ratio is lowered to provide the best power output while controlling detonation and overheating of the cylinder. The VCR engine can continuously vary the compression ratio, so the thermodynamic benefits appear throughout the engine load range. Thus the VCR engine provides the best of both worlds; fuel efficiency with lower emissions, while providing maximum power output from the engine. All internal combustion engine aspirations, Naturally Aspirated (NA), Turbo Charged (TC), and Super Charged (SC), can benefit from VCR technology. Additionally VCR engine technology will be needed in order to enable the Homogeneous Charge Compression Ignition (HCCI) engine. HCCI is a form of internal combustion in which a well-mixed air/fuel ratio is compressed to the point where the fuel autoignites. This autoignition is much like a diesel engine except it uses gasoline as the fuel stock.
Not just a theory – in production!
Let’s look at the first production VCR engine in use. This engine was designed by the Nissan motor group. It uses a Multi-Link Rod-Crank style VCR system as seen in Figure 3. The first observation is that the connecting rod is no longer directly connected to the crankshaft, but instead is connected to a multilink assembly. This linkage assembly is connected to a control rod that is connected to an eccentric shaft. When this eccentric shaft is rotated with a computer controlled gear reduction electric motor, the control rod changes the geometry of the multilink assembly. In one position, the eccentric shaft rotates so the control rod is raised allowing the multilink assembly to move downward on the opposite end so the piston height within the cylinder is lower, lowering the compression ratio. In another position, the eccentric shaft rotates so the control rod is lowered allowing the multilink assembly to move upward on the opposite end so the piston height within the cylinder is higher, raising the compression ratio. This linkage system changes the piston height approximately 6 mm, changing the static compression ratio from 8:1 to 14:1 in about 100 ms of time.
This system is used on the 2018 Infiniti QX50, and is shown in Figure 4. The engine is a 2.0 liter turbocharged four-cylinder VCR engine that produces 268 hp (200 kW) @ 5,600 rpm and 280 lb ft (380 Nm) @ 4,400 rpm. The result of this is an engine that gets 27 percent better fuel economy than Nissan’s 3.5-liter V6, at roughly the same HP and torque, but is smaller and lighter. Perhaps this Multi-Link Rod-Crank style VCR system is the best design from the view point of mass production.
More VCR concepts
In Figure 5 an Eccentric Bearing style VCR engine is shown. This VCR engine has the main bearing bore designed so that this bore is off center. The main bearings are then supported by an additional bearing set so that the main bearing assembly floats in the engine block. A control lever is attached to the floating main bearing, which is connected to a control link that in turn is connected to a control lever arm assembly. This control lever arm assembly can be rotated by a computer controlled electric motor with gear reduction. In one position the main bearing is rotated so the piston height is lowered in the cylinder bore, lowering the compression ratio. In another position the main bearing are rotated so the piston height is raised in the cylinder bore, raising the compression ratio.
In Figure 6 a Hydraulic Connecting Rod VCR style engine is shown. This VCR engine has a more conventional look where the connecting rod attaches to the crankshaft and piston. However, the piston end of the connecting rod is much bigger due to the hydraulic control pistons and articulating piston pin assembly. The articulating piston pin assembly has the piston pin hole offset. When the hydraulic pressure is applied by the computer control valve to one of the control pistons, this assembly moves the position of the piston pin so that the piston is moved downward, lowering the compression ratio. When the hydraulic pressure is applied to the other hydraulic control piston the piston pin assembly is rotated so as the piston is moved upward, increasing the compression ratio.
Figure 7 shows an additional piston volume change VCR style engine. This style VRC engine was the first VCR engine that was built, and was used for rating the octane of gasoline. It was designed by Harry Ricardo in the 1920s. This engine design has a much more conventional look to it. The main difference is the volume piston contained in the cylinder head. With the volume control piston in the upward position the clearance volume is increased, lowering the compression ratio. When the control volume piston is moved in the downward position the clearance volume is decreased, increasing the compression ratio.
Testing a VCR engine
Now that you understand the inner workings of the VCR engine, it will be quite easy to test. You will need a scan tool, oscilloscope and a pressure transducer. Install the pressure transducer in the cylinder head in place of the spark plug. Now start the engine and, without changing the throttle or RPM (which would change the volume), use the scan tool to command the VCR system to change the compression ratio. The pressure should increase or decrease with the VCR commanded ratio change. The pressure change will be directly related to the static compression ratio that the engine can obtain. If the engine has a compression sensor in the combustion chamber you can match the sensor output reading against the pressure transducer reading. Now you will be ready for these high tech engines when they roll into your service bay.
