Compression Ratio

5-06 Relate compression ratio to engine application.

Static or base compression ratios are fixed, established during the design and subsequent engine-building process. The compression ratio is the difference between the cylinder volume with the piston at BDC and the volume with the piston at TDC. Increasing the compression ratio increases the heat developed in the cylinder, further increasing the compression ratio during the compression stroke (FIGURE 5-19).

FIGURE 5-19 The compression ratio of an engine is found by taking the volume of the cylinder at BDC and comparing it to the volume at TDC. In this example, a 9:1 compression ratio is found.

Use the following formula to compute the compression ratio:

The combustion chamber is simply the cavity (space) in the cylinder head casting above the piston when the piston is at TDC. An exception to this rule is a bowl-in-piston design, typically found in many diesel engines. Depending on engine design, the top of the piston can be flat and level with the top of the cylinder block at TDC or dome-shaped (protruding past the upper part of the cylinder block). When the piston is at TDC, this is the smallest dimension in the above formula. When the piston is at BDC, it is the largest dimension. The design of the combustion chamber directly affects the engine’s efficiency to flow air through the engine, or “breathe.”

A compression ratio listed as 10:1 (10 to 1) means that the maximum cylinder volume is 10 times larger than the minimum cylinder volume. Changes can be made to alter an engine’s static compression ratio, including altering the size and shape of the top of the piston, altering the size of the combustion chamber, or altering valve timing.

The compression ratio is crucial to engine design and affects engine performance and fuel economy. Increasing the compression ratio by one point (e.g., from 9.0:1 to 10.0:1) will increase the fuel economy on most vehicles by approximately 1 mile per gallon (mpg), or 235 liters per 100 kilometers (L/100 km). Compression ratio is also theoretically directly related to an engine’s horsepower (wattage) output. The higher the compression ratio, the more power the engine is capable of producing (assuming the rest of the engine design does not inhibit output). Higher compression is beneficial to manufacturers, because a smaller displacement engine is capable of producing horsepower equivalent to a larger, heavier low-compression engine. As engine size diminishes, attaching components follow a size reduction also, increasing weight reduction and improving performance and fuel economy.

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A square engine has the same size bore and stroke. An engine with a larger bore than stroke is an oversquare engine (short-stroke engine). Undersquare engines, use a bore smaller than the stroke (long-stroke engine). Oversquare engines, tend to make their power at higher rpm, while under-square engines tend to make their power at lower rpm.

A limit exists when increasing compression ratios before engine damage can occur. By exceeding the maximum compression limit, the result could be igniting the air-fuel mixture before it is required, causing spark knock, or detonation. Any one of these occurrences is detrimental to and can reduce engine life. Typically, a compression ratio of 9.3:1 to 10.0:1 (conventional engine) is the maximum that is allowable on 87-octane fuel. Octane is the measurement of how much compression that fuel can resist before igniting. Fuel with a lower octane rating burns faster than fuel with a higher octane rating. The higher the octane rating increases (higher octane number rating), the less likely the fuel is to pre-ignite (explode unexpectedly) at higher pressures and potentially damage the engine. Raising the octane level of the fuel slows the burning process in the combustion chamber. Increasing the octane level enhances the fuel’s ability to withstand greater pressures without pre-igniting. As compression ratios continue to rise, the octane rating of the fuel used should also increase. Manufacturers provide the recommended fuel octane level in the owner’s manual.

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Two forms of uncontrolled combustion events can occur: detonation or pre-ignition. The two are completely different, and both occur at various times in the combustion process. Normal combustion is not an “explosion” but rather a controlled burn process that begins at the spark plug and spreads throughout the combustion chamber.

  1. Detonation is an instantaneous, erratic form of combustion that typically occurs under acceleration or heavy load, described as spark knock. Detonation is a secondary ignition event that begins independently of the spark created from the spark plug. An unburned air-fuel mixture remains (end-gas) in the cylinder, beyond the boundary of the propagating flame front generated by the spark plug. Subjecting this unburned air-fuel mixture to a combination of heat and pressure in the combustion chamber for a specified duration (beyond the ignition delay period of the fuel or octane rating) may cause detonation to occur. This unwanted ignition event creates multiple flame fronts that collide, creating severe, short, localized shock waves with increasing cylinder pressures and temperatures. The collision of flame fronts causes a heavy pinging or knocking sound. Detonation always occurs after normal combustion of the air-fuel mixture in the cylinder. Consistent detonation over an extended time may lead to engine damage.
    Note: Mild detonation can occur intermittently in any engine and is not cause for concern. If detonation increases in severity or frequency, the cause and recommended repair will need to be diagnosed.
  2. Pre-ignition occurs anytime something causes the fuel mixture to ignite before firing the spark plug.

Valve Overlap and Scavenging

Valve overlap occurs when both valves are open as the piston travels in the bore. The valves, although open at the same time, do not contact each other. Valve overlap is ground (designed) into the camshaft and is not adjustable on engines without variable valve timing (VVT).

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Valve overlap occurs when both the intake and exhaust valves are open at the same time, between the exhaust and intake strokes. Reducing valve overlap restricts breathing efficiency, which is particularly noticeable at high rpm, but it creates a stable idle. Increasing valve overlap enhances engine breathing, improving VE, but it results in poor idle quality.

Valve overlap begins as the piston approaches TDC on the exhaust stroke and continues until just ATDC. The correct overlap is essential for optimal performance, encompassing endless hours of engineering design. When both valves are open simultaneously, a drop in cylinder pressure occurs. Overlap is the ability of intake and exhaust flows to affect each other due to pressure waves that vary with load and engine rpm. Overlap is well balanced at TDC on most engines. Piston motion has the greatest effect on airflow the further from TDC that valve overlap exists.

The theory behind valve overlap is that since the intake valve is open slightly, departing exhaust gas flow creates a pressure differential (vacuum) that aids in pulling fresh air-fuel mixture into the cylinder, without any of the intake charge passing into the exhaust system (scavenging). It also aids in the fresh air charge helping to displace the leftover exhaust gases that remain in the cylinder.

As the piston travels from BDC, pushing exhaust gases from the cylinder through the open exhaust valve, the intake valve begins to open before the piston reaches before top dead center (BTDC). As the piston approaches BDC on the intake stroke, the pressure differential of the cylinder is almost equal; however, the air-fuel mixture continues to fill the cylinders a small amount, enhancing airflow. As air flows into the cylinder, it gains velocity, creating a column of air contained within the intake port and manifold runner. This airflow is column inertia. The cylinders continue to fill due to the inertia present in the air-fuel mixture that is already moving. This principle of airflow allows the intake valve to be held open as the piston is starting its climb back up the cylinder. The exiting exhaust gases increase the intake charge exceeding that, which would usually enter the engine from piston travel alone. Valve overlap also helps purge the spent exhaust gases by the incoming fresh air charge until the exhaust valve seats. Delaying the intake valve from closing allows the cylinder to pack as much of the air-fuel charge into the cylinder as possible. Inertia causes the air molecules to pack together more tightly, especially at higher rpm, thus creating a ram-air effect. Think of this as an inferior turbocharger/supercharger.

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Inertia: a property of matter in which it continues in its existing state of rest or uniform motion in a straight line unless changed by external forces. In other words, a body that is in motion stays in motion; a body at rest stays at rest.

Increased overlap is beneficial at higher speeds and loads due to exhaust pressure waves drawing in the intake charge while both valves are open. Large overlap at lower speeds results in poor emissions as the mixture flows directly into the exhaust, never burning. At idle, increasing valve overlap produces a rougher idle by pulling slowly moving exhaust gases back into the intake manifold, diluting the incoming fresh air-fuel charge. High overlap also results in an EGR effect that although beneficial for emissions at part load, reduces power under full load. Improved exhaust scavenging and induction system (intake) breathing work together to improve VE. Improving breathing is achieved by smoothing intake and exhaust passages, using tuned intake and exhaust runners (to maximize ram effect and scavenging) and a low backpressure exhaust.

A lack of valve overlap will reduce engine performance at higher speeds while maintaining excellent idle and low-speed operation. Trucks typically have a minimum of valve overlap to increase low-speed torque. Passenger cars, however, must balance the amount of valve overlap present, depending on vehicle use.

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Scavenging is the process of using a column of moving air to create a low-pressure area behind it to assist in removing any remaining burned gases from the combustion chamber and replacing these gases with a new charge. As the exhaust stroke ends and the intake stroke begins, both valves are open for a short time. The time that both valves are open is valve overlap. As the exhaust gases leave the combustion chamber, the flow tends to continue, creating a low pressure behind it that helps to draw the intake air-fuel charge into the cylinder. At the same time, the movement of the air-fuel charge being pushed (by atmospheric pressure) into the combustion chamber also helps to push the remaining exhaust gases out. The flow effect during this valve overlap is scavenging. The rpm at which the most efficient scavenging occurs contributes to peak VE and engine peak torque.

The following are typical overlap ranges of crankshaft rotation for various applications: trucks/good mileage towing 10–35 degrees; daily driven low-rpm performance, 30–55 degrees; hot street performance, 50–75 degrees; bracket/oval track racing, 70–95 degrees; dragster/comp eliminator engines, 90–115 degrees.

Volumetric Efficiency

The VE of a four-stroke engine is defined as the difference between how much air-fuel mixture actually enters the cylinder with the engine running and the total cylinder volume (bore × stroke). It is a rating of the engine’s ability to breathe (fill the cylinders): the airflow throughout the engine. Stated another way, it is a measure of the volume of air actually occupying the cylinders at any given engine speed divided by the physical volume (displacement) of the engine expressed as a percentage. This percentage describes how full the cylinders are. The higher the percentage, the fuller the cylinder is and the more efficient the engine is. VE affects engine performance directly: a cylinder that is completely full of a fresh air-fuel charge will produce more power than a cylinder that is only half full.

The following engine design characteristics affect VE:

Valve overlap has a significant effect on cylinder filling or VE and is ground into the camshaft, and only is variable with camshaft replacement.

Using forced induction further increases VE. Turbochargers and superchargers force more air into an engine than can enter a conventional, naturally aspirated engine. Forced induction allows a smaller, lighter-displacement engine to mimic the power output of a larger, heavier-displacement engine.

Theoretical Four-Stroke Cycle Engine Valve Timing and Valve Overlap

Proper combustion and engine power require precise control of valve timing. The camshaft design matches the ideal valve opening and closing events to the crankshaft position and rotation. Optimal valve timing increases airflow into and out of the cylinders. Altering the base relationship between the crankshaft and camshaft affects VE. Valve timing enhances or reduces engine performance and VE.

As stated earlier, VE and power output increase when more air, and therefore more fuel, is forced into the combustion chamber. Power and VE continue to increase with additional airflow up to the point at which the physical limitations of the engine have been met or exceeded. These limits include intake runner dimensions, cam lift and duration, intake valve diameter, the exhaust system’s ability to remove air, and combustion chamber design.

When the piston is sitting at TDC and preparing to go down, the intake valve opens and air is drawn in with the fuel (unless the engine is direct injected, in which case it is just air). As the cam allows the valve spring to close the intake valve at BDC, the piston continues moving toward TDC, squeezing the air and fuel into a smaller, tightly packed mixture. Sometime near TDC, the spark plug ignites the air-fuel charge. Igniting the fuel drives the piston back down to BDC. After the power stroke has completed, the exhaust valve opens with the piston at BDC, and then the piston begins moving back to TDC, pushing the exhaust out of the cylinder.

In reality, the valves open and close sooner or later than in this description. Because of inertia and other forces, the valves need enough open time to allow gases to flow in and out of the engine. If the valves opened and closed as described in the preceding paragraph, the valves would have 180 degrees of cam duration (TDC to BDC and BDC to TDC). Although this would be sufficient for low-rpm engines, it would not work for high-rpm engines. Thus, the intake stroke begins by opening the intake valve as the piston is still moving up and the exhaust valve is still slightly open but closing (FIGURE 5-20).

FIGURE 5-20 The basic Four-Stroke cycle.

As discussed earlier in the valve overlap section, as air flows into the cylinder, it gains velocity, which creates a column of air that has inertia and keeps the airflow moving, or racing, into the cylinder. If the intake valve is held open after BDC, air will continue to rush in because of the inertia of the airflow, which keeps air flowing until its inertial energy has been spent. Changing exhaust valve timing aids in cylinder filling through scavenging, reduces the work required to raise the piston against pressure, and (blow down pumping work) provides internal EGR.

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Igniting the air-fuel mixture early enough allows ample time for it to burn com-pletely, producing the desired power. The air-fuel mixture has a relatively constant burn time, so as engine rpm increases, the spark will have to occur earlier (in degrees) BTDC to make sure the maximum pressure is developed shortly ATDC. Attaining maximum power results when a factor known as location of peak pressure (LPP) occurs at 14 degrees ATDC.

Intake Valve Opening
  • By opening the intake valve before TDC, exhaust gases may flow back into the intake manifold instead of exiting the cylinder via the exhaust valve and port.

    •  Exhaust gas reversal into the cylinder results in internal EGR.

    •  EGR is detrimental to full-load performance, taking up space that would otherwise be occupied by a fresh intake charge.

    •  Internal EGR may be beneficial at part-load conditions for both emissions and engine efficiency.

  • Opening the intake valves later provides more of a restriction to the entry of the air-fuel charge from the manifold. Late IVO causes in-cylinder pressures to drop as the piston starts to descend after TDC.

    •  Retarded (late) intake valve opening (IVO) can also result in internal EGR by holding the exhaust valve open, allowing exhaust gases to be drawn back into the cylinder by scavenging.

  • The exhaust valve closes to reduce internal EGR, preventing exhaust gases from entering the cylinder when delaying IVO.
Intake Valve Closing
  • Maximum torque requires closing the intake valve at the point where the greatest mass of the fresh air-fuel mixture is held in the cylinder.
  • Pressure waves in the intake system normally result in airflow into the cylinder after BDC due to inertia.
  • Consequently, optimal intake valve closing (IVC) timing changes considerably with engine rpm.
  • The ideal IVC timing moves further after BDC in an attempt to gain the maximum benefit from the intake pressure waves.
  • When closing the intake valve either before or after the optimal timing for maximum torque, a lower mass of air gets trapped in the cylinders.
  • Early IVC reduces the mass of air that can enter the cylinder.
  • Closing the intake valve late allows air that is inside the cylinder to flow back into the intake manifold.
  • By closing the intake valves either early or late, intake pumping losses are reduced, improving part-load efficiency.
Exhaust Valve Opening
  • To maximize work (efficiency) from the expansion of the gases in a cylinder, do not to open the exhaust valve before the piston reaches BDC.
  • The pressure in the cylinder should be allowed to drop to the lowest possible value (exhaust backpressure) before the piston begins to rise.
  • Low cylinder backpressure reduces the work done by the piston in expelling the products of combustion (blow down) before the intake of the fresh charge.
  • The following are obviously two conflicting requirements of exhaust valve opening (EVO):
  1. Requiring EVO to be ABDC.
  2. Requiring EVO to occur BBDC.
Exhaust Valve Closing (EVC)
  • The timing of the EVC has a very significant effect on the amount of exhaust gas that remains in the cylinder at the start of the intake stroke.
  • During part-load operation, retaining some the exhaust gases may be beneficial, reducing the amount of fresh air-fuel charge that can enter the cylinder.
  • Retained exhaust gas is inert. Therefore, it cannot produce power, resulting in a lower engine rpm.

    •  The lower rpm reduces the for the throttle plate to restrict airflow into the intake.

    •  Increasing the throttle plate opening lowers pumping losses.

  • Moving the EVC timing further ATDC increases the level of internal EGR, resulting in a reduction of exhaust emissions, specifically oxides of nitrogen (NOx).
  • For full-load operation, the opposite holds true: Ensure that the minimum possible quantity of exhaust gases is retained in the cylinder to allow the maximum volume of fresh air-fuel charge to enter the cylinder during the intake stroke.

    •  This requires the EVC to be at TDC or shortly ATDC.

  • Engines with an exhaust system that is relatively active (pressure waves are created by exhaust gas flow from the different cylinders), EVC timing influences whether pressure waves in the exhaust are acting to draw out exhaust gas from the cylinder or to push exhaust gas back into the cylinder.

Consider again the definition of valve overlap described earlier. In the real world, even standard camshaft grinds (valve timing) include some degree of valve overlap at BDC and TDC. In traditional engine designs, camshaft design and valve timing are a compromise for street engine use. Racing engines are designed with greater valve lift, longer valve duration (valve open time in degrees of camshaft rotation), and increased valve overlap to maximize the effect of fuel and air inertia encountered at higher rpm.

At cranking speeds, there is not much column inertia, so having the valve open for a long time would reduce engine intake vacuum and make the engine difficult to start because the intake air would move back out past the intake valve as the piston travels up on the compression stroke. With no column inertia, leaving the intake valve hanging open during the initial portion of the compression stroke will lower engine compression.

The other problem that can result with large overlap is that at cranking speeds, the intake will be opening during the exhaust stroke, allowing exhaust gases that are still burning to work their way out the intake valve and create a backfire in the intake manifold (not a desirable effect). When the engine starts and the speed of airflow therefore increases, column inertia begins to increase, reducing the problems experienced while cranking.

The physical shape of the cam lobes controls the valve train. The entire cam can be moved forward or backward in relation to the crankshaft position, making the events happen sooner or later during the piston stroke. Do not confuse advancing or retarding cam timing with ignition timing, which is when the spark occurs. Altering cam timing requires using a degree wheel to set the timing before installing the timing chain or belt. The limitations of base cam timing have resulted in the use of VVT, overcoming the inefficiencies of basic cam timing.

The following options for adjustments are available to change cam timing in a fixed valve engine:

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Alignment between the cam gear(s) to the crankshaft gear affects cam timing. The mechanical relationship that is present between the two can alter cam timing by advancing or retarding the camshaft timing. Gear wear on any sprocket, excessive backlash, and belt or chain stretch can all alter this relationship and therefore affect cam timing. Resurfacing an OHC cylinder head may also change cam timing and may require a selective fit head gasket or an offset cam drive pulley to compensate for the modification.

  • Crankshaft and camshaft gears can be installed by using selectable offset dowel pins or keyways.
  • An adjustable camshaft gear(s) can advance or retard the cam timing.
  • Some performance timing sets use an adjustable cam gear for ease of indexing in different positions so that the cam can be advanced or retarded in 2-degree increments to gain additional power at the desired operating rpm range.

Advancing cam timing closes the intake valve sooner, increasing cranking compression again, because of low column inertia. Advancing the camshaft creates more torque and power in the low-rpm range. If cam timing is retarded, then the intake valve closes later and provides more torque and power at a higher rpm range. Column inertia is greater at high rpm, allowing additional air to enter the engine when the valve is open later in the four-stroke cycle. Cam duration (valve open time) is built into the cam lobe profile and usually cannot be changed. Duration determines how long the valves remain open or closed. Advancing or retarding the cam opening or closing time affects how long the valves stay open or closed. The point at which they open and close in the cycle is designed for a distinct engine purpose: peak torque rpm range. Turbocharged and supercharged engines do not typically use aggressive valve overlap.

Typically, when camshaft timing is off, it retards due to timing component wear or breakage. Retarded timing presents as an unstable idle, stalling, low engine manifold vacuum, low power at low rpm, and possible misfire(s). A concern that is becoming more apparent is timing chain stretch. Few manufacturers offer a specification for timing chain stretch. When a chain or belt stretches, it retards cam timing and causes the tensioner to continue to extend, reaching the tensioner’s adjustment limit. Most manufacturers that support cam timing error may set a code for cam timing that is 10 degrees out; this is the approximate equivalent of about two-thirds of an average chain or belt tooth.

If the cam timing fails in the advanced state, it is usually from an installation error either during initial setup or because the belt/chain has been set too loose, which may allow the belt/chain to jump (this can occur when cam timing is either advanced or retarded). When advancing cam timing, idle quality will remain unaffected: low- to mid-rpm performance may fail to prove a fault, but high-rpm performance and power will suffer.

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An extended tensioner may indicate a stretched timing chain; however, a dam-aged or missing chain guide will also cause the tensioner to “max out.” A thor-ough visual inspection of the timing chain components will provide an indication of timing chain stretch. If all the timing components are present and unbroken, a maxed out tensioner is a good indica-tor of timing chain stretch. Any missing or damaged parts, along with the tensioner and chain(s), will need to be replaced to prevent future issues and failures.

Engine Measurements

The following information on engine design can and does affect engine performance. While not typically “adjustable” for an engine performance problem, this information can and will provide a working knowledge of design characteristics that contribute to engine design and how their interaction is used to achieve the balance between developing power, drivability, and intended usage. Thoroughly understanding these performance characteristics should prove beneficial to technicians.

Displacement

ICEs are designated according to the amount of space (volume) their pistons displace as they move from TDC to BDC, which is called engine displacement. Therefore, a 5.4 L V8 engine has eight cylinders that displace a total volume of 5.4 liters. Displacement’s measurements include cubic centimeters, liters, or cubic inches. To determine an engine’s displacement, find the bore, stroke, and number of cylinders for a particular engine (FIGURE 5-21).

FIGURE 5-21 Piston displacement.

The diameter of the engine cylinder is the cylinder bore. Bore measurement is across the cylinder, parallel to the block deck, which is the machined surface of the block farthest from the crankshaft. Automotive cylinder bores can vary in size from less than 3" (7.6 cm) to more than 4" (10 cm).

The distance that the piston travels from TDC to BDC, or from BDC to TDC, is the piston stroke. The offset portion of the crankshaft, or the throw, determines piston stroke. Crankshaft description occurs in detail later in this chapter. Piston stroke also varies from less than 3" (7.6 cm) to more than 4" (10 cm). Generally, the longer the stroke, the greater the engine torque produced. A shorter stroke enables the engine to run at higher rpm to create greater horsepower (wattage). Engine specifications typically list the bore size first and the stroke length second (bore vs. stroke).

The volume that a piston displaces from BDC to TDC is piston displacement. Increasing the diameter of the bore or increasing the length of the stroke will produce a larger piston displacement. The formula for calculating piston displacement is cylinder bore squared × 0.785 × the piston stroke.

This formula works for calculating either the imperial displacement in cubic inches or the metric displacement in cubic centimeters (ccs) or liters. For example, a 5.4 L (329 cubic inch) V8 truck engine has a 3.55" bore, a 4.16" stroke, and eight cylinders. Use the following formula for displacement:

3.55 × 3.55 (bore2) = 12.6025 × 0.785 (constant) = 9.893 × 4.16 (stroke) = 41.155 cubic inch piston displacement

Once the piston displacement has been calculated, the next step to finding engine displacement is to multiply piston displacement by the number of cylinders in the engine.

The previous example continues here: 41.155 cubic inch piston displacement × 8 (number of cylinders) = 329.24 cubic inch engine displacement.

To alter an engine’s displacement (engine size), change the cylinder bore (diameter), piston stroke (length), or the number of cylinders.

Bore and Stroke

The bore of a cylinder is its diameter, in which the piston travels, measured in either inches (”) or millimeters (mm). Stroke is the distance a piston travels between TDC and BDC. The sum of a cylinder’s bore and stroke determines an engine’s displacement. To create a balanced and powerful engine, bigger bore and stroke ratios are often used.

An engine with the same size bore and stroke is a square engine. These engines typically have a bore and stroke ratio of approximately 1:1.

An oversquare engine has a bigger bore than stroke, giving a ratio greater than 1:1. Oversquare engines are the most common in current automotive use. Oversquare engines tend to make their power at higher rpm due to shorter piston strokes and are typically found in Formula One racing cars and motorcycles. Due to the shorter stroke piston, speed slows. Large bore engines also use bigger valves to improve airflow volume, increasing their breathing ability.

An engine with a bore smaller than the stroke is called an undersquare engine (long-stroke engine), giving the engine a ratio value of less than 1:1. Undersquare engines tend to make their power at lower rpm and rev slower. These engines produce strong torque output in the low- to mid-rpm range due to their longer rod throw. The smaller bore limits the number of valves and size of the valves.