Engine Combustion Analysis by isystems performance

Combustion Analysis: A better way…

The dynamometer is an important tool, but horsepower is calculated from torque. Torque is the key measurement – typically the ultimate performance measurement for most engine builders.

But is it really a single number? Not at iSystems – there is more…

Combustion analysis gets you more by monitoring internal combustion engine variables. We do this every day utilizing specialized pressure transducers, allowing us to monitor cylinder pressure as the crankshaft rotates. At iSystems we actually watch combustion happen in each cylinder, which makes the path to greater performance clear.

Everything about the internal combustion engine is about burning fuel to create pressure to push the piston. Don’t believe us?

Let’s deconstruct some data… and see the good, the bad, and the ugly.

Since each cylinder has its own pressure transducer, each can generate multiple permutations depending on how the pressure is monitored. Examples include: average cylinder pressure, Peak cylinder pressure variation, 50% burn variation, and IMEP variation, just to name a few.

Each data type highlights a different aspect of the overall combustion process with the motor. Analyzing the "data suite" allows "picking apart" the combustion process to get at what is truly amiss.

So let's look at a simple case.

The most intuitive data set to visualize is the Average Cylinder Pressure. This set was collected from a two-liter Porsche flat six at 3500 rpm.

The first question of true science (rather than guessing) is… What do we observe?

What is obvious?
• Some cylinder traces don’t overlay. A 50 psi variation… 20% of the average!
• Pressure continues to rise after passing 0° (TDC), yet cylinder volume is increasing.
• All the cylinders but for #3 achieve peak pressure at the same crank angle – 22 vs 28°.
• The rate of rise of the peak increases after ~15° BTDC… ignition timing is set at 16°.
• Traces really begin to deviate after ignition.

What are these observables telling us?

Pressures before ignition are tightly grouped, but pressures after ignition are not. We know fuel burns at a fixed rate, so cylinder #3 peaking late suggests ignition timing. Late ignition on #3 would also contribute to lower peak pressure.

Remember, the dynamometer itself only gives us an AVERAGE of engine response, in a single number, so this would never be observed!

Okay, still with us? Let’s discuss where the bigger performance payoff lies. It’s a little secret that the engineers don’t tell you. They detune their engines… on purpose.

Why?

It’s hard to beat Mother Nature. We think of mechanical devices as repetitive, clockwork things. One cycle should be the same as the next. However, the fuel mist and air flowing into the engine follows stochastic behavior - random chance rules. This means the fuel air mixture around the spark plug varies and doesn’t "light off" the same every time. This small variation has a pronounced effect on the burning of the bulk of the fuel air charge. If the ignition is slow to catch, low cylinder pressures are observed. However, if the ignition is optimal, high cylinder pressures are observed – high enough to turn into detonation. Once again let’s look at real data. This time it is the crank position when half of the fuel air charge is actually burned. It is observed over 75 combustion cycles. Notice that our event wanders over 16 crankshaft degrees in this example. This is the same as having your ignition system wander between advance and retard over 16 degrees!

But the real-world result is that engineers detune their designs so that the random, really good ignition events don’t leave you on the side of the road.

They have to deal with production line tolerances. You don’t. iSystems can help. We can show you how the engineers have cheated you. We can show you what needs to change to smooth out the peaks and valleys, allowing you to tune for that extra power hidden in your engine – safely. We can also design the parts to execute these changes, as well as assemble the engine with the care required for optimal operation.

Combustion Analysis has primarily been a research tool. Not anymore.
iSystems can put its power to your use. Remember, a dynamometer only gives you a WHOLE ENGINE average. With Combustion Analysis you get data at each and every crankshaft degree. Data is measured and collected with the application of Kistler Instrumentation, A&D Technology systems and AVL encoder.