Knowing that engines produce power by harnessing the dramatic pressure increases resulting from the ignition of the air/fuel mix, let’s take a look at how that pressure plays into the calibration requirements. Cylinder pressure is directly related to engine efficiency and output. Engineers often refer to BMEP, or brake mean effective pressure, to describe engine operation. Throttling airflow and changing ignition advance have the biggest impact on BMEP, but cam timing, manifold pressure, and lambda also play a large part in determining cylinder pressure. The underlying concept is that higher cylinder pressures generally indicate a more efficiently running engine under normal operation. More efficient combustion requires less ignition lead. If the calibrator can recognize where the engine is most efficient, it becomes easier to determine ignition needs.
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If one were to look at a map of actual engine efficiency and compare this to the ignition map showing MBT timing, the relationship between cylinder pressure and required advance becomes evident. Conversely, we can employ this knowledge to use ignition advance to increase cylinder pressure (and torque) in areas where the engine is less efficient. Advancing ignition timing at low RPM under load can help improve throttle response before the camshaft and intake runner efficiency come up in the midrange. Likewise, at higher RPM after the engine passes the peak efficiency range of the cam and intake runners, cylinder pressure begins to fall off. More power can be made by increasing ignition advance to make up for the decreasing engine efficiency as the engine approaches redline.
Calibrators must be keenly aware of two unique conditions of increased cylinder pressure: Knock and preignition have very similar symptoms and results with slightly different causes. Knock is caused by an uncontrolled fast burn in the chamber after the initial spark event. This is often the result of excess ignition advance, lean mixtures, or elevated temperatures. Preignition is the premature start of combustion before the normal spark event. This is often caused by hot spots in the combustion chamber or residual glowing hydrocarbon deposits that ignite the mixture before the spark plug has a chance to fire. Either way, both conditions mean that peak combustion pressure has been reached before the piston arrives at TDC. This means that not only is normal combustion pressing on the cylinder head, gasket, and piston, but dynamic compression happens as well. The symptom for this condition is usually a sound similar to marbles in a coffee can. While not always audible, the intensity of this noise grows with the intensity of engine stress resulting from the condition. The forces associated with knock and preignition can be as much as two or three times the normal combustion forces. This can be incredibly destructive to engine components, often resulting in a blown head gasket, broken piston, bent connecting rod, or failed bearings. Knock should be avoided, period.
Burn rate of the air/fuel mixture has a dominant impact upon just how much ignition lead an engine wants. The speed at which the combustion process occurs directly influences the rate of cylinder pressure rise.
Most spark ignition engines have an effective flame propagation speed of 706 to 985 in/s (18 to 25m/s). This means that a flame starting from a central spark plug in a 4.00-inch bore engine can reach the cylinder wall in about two and a half milliseconds. At an idle speed of 800 rpm, this translates into roughly 11.5 degrees of crankshaft revolution. That same flame speed and bore size at 3,000 rpm yields more than 43 degrees of crankshaft rotation during our theoretical burn time to reach the chamber walls. This is why ignition timing must advance at higher speeds to ensure combustion before the piston has a chance to run away from the reaction.
Actual flame propagation speed changes with conditions. Faster flame speeds and better charge mixing during combustion speeds up the reaction that is producing the expanding gases and rising pressure. Burn rate is influenced by a number of engine operating conditions beyond just chamber design.
Fuel octane is a number representing the ratio of octane to heptanes in gasoline. Higher octane numbers indicate a stronger concentration of the slower, more stable- burning octane molecules than the more volatile heptane. Since high-octane fuels have a slower burn rate, larger amounts of ignition lead can and should be used. Increasing the ignition lead results in an increase in torque, so higher fuel octane levels allow for more power resulting from the necessary ignition advance increase. Keep in mind that fuel octane is a global change to the engine’s ignition advance needs. Running higher octane fuel means more spark advance at idle and part load as well as WOT. Changing from 93 octane pump fuel to 110 octane racing fuel may allow 4 degrees of additional spark advance at
WOT (with an accompanying increase in power), but may idle poorly and exhibit reduced fuel economy and throttle response unless all other spark tables are increased by a like amount.
Lambda changes burn rate, often requiring an adjustment to commanded ignition advance. Slightly rich (l » 0.9) mixes tend to burn quickest, requiring less ignition advance to reach peak cylinder pressure at the right time. Base and MBT ignition tables are usually calculated for λ = 1 for OEM applications with a second table to adjust for changes in lambda. These tables are typically a global ignition adder to increase ignition advance whenever an excessively rich condition is entered. This helps compensate for the changes in burn rate as well as keep exhaust gas temperatures within an acceptable range.
Charge temperature has a similar effect on burn rate. Hotter intake charges more readily evaporate fuel and increase the burn rate. Much like lambda, the OEM solution usually involves an adder function to reduce global timing with higher inlet temperatures. For this to work properly, the IAT sensor must be installed such that its output is directly proportional to actual inlet temperature to the cylinder. Installation of a supercharger or large amount of plumbing between the IAT sensor and the intake port skews the reading and reduces accuracy of this compensation curve. If the IAT is installed after a supercharger, it has the benefit of allowing the PCM to adjust timing to compensate for heat soak in the compressor, improving safety margin against detonation at high temperatures.
Engine load is an indicator of final charge density in the combustion chamber and ultimately reflects engine efficiency. This charge density increase can come from an ambient barometric condition, throttle position, artificial increase in manifold pressure (supercharging), or higher compression ratio. The result is a mixture of more closely packed molecules that allow the flame front to easily hop from one molecule to the next in the chamber. These more efficiently packed molecules burning at a faster rate again require less ignition timing advance to reach peak pressure at the proper moment.
Mechanical mixing of the air/fuel charge inside the cylinder helps to increase bulk burn rate. Although actual flame speed remains relatively constant, the tumble and swirl motions inside the chamber move the mass of burning charge around exposing it to unburned mass even more quickly. The result is a faster, more complete combustion event.
“Balancing the Players”
When it comes to increasing or maintaining cylinder pressure, we now see multiple ways to get to the desired output. If an engine can only tolerate a given amount of cylinder pressure, the calibrator often finds himself with several options to achieve that pressure. There is a balance that needs to be preserved between lambda, spark advance, load, and temperatures. This is especially true at WOT where the same power may be developed using a lean mixture with less timing or a rich mixture with increased advance. It is up to the calibrator to decide which
balance to strike. There is no free lunch at the limit. If an engine has reached its knock limit at λ = 0.82 with 26 degrees of advance, we must either increase fueling to add ignition lead or reduce timing to find power with a leaner mix.
At cruise, the balance is simpler because we are usually targeting λ = 1. In this case we simply find the ignition lead that develops MBT (highest thermal efficiency) and allow for adjustment for environmental changes. Since the engine is operating in a rang where flame speed is slightly slower than under full power, some additional timing must be added to optimize combustion. Many new calibrators are surprised by just how much ignition advance an engine requires for best operation at light load and cruise conditions compared to full power operation.
Written by Greg Banish and Posted with Permission of CarTechBooks