Gasoline Engine Exhaust Manifold Temperaturesby Richard Rowe
Exhaust gas temperatures are telling signs of internal engine function, and can deliver much-needed information regarding combustion efficiency. And it goes further than that: high EGTs can melt aluminum components and warp those made of steel or iron. Whether you're running gas or diesel fuel, keeping an eye on the EGT is one sure way to keep your vehicle's engine running safely and efficiently.
Exhaust gas temperatures rise or fall based largely upon the air/fuel ratio, but how the air/fuel ratio affects the EGT depends on the engine itself. Diesel engines work by squeezing the air/fuel mixture until it heats up to the point of ignition, whereas gas engines set the mixture off with a spark. The spark ignition allows pressure in the cylinder to get closer to its peak before the ignition event, resulting in a much quicker burn. Oxygen ends up as the limiting reactant in a gasoline engine because the fuel burns so quickly, which is why gas engines control rpm by metering airflow. The diesel engine's much slower burn means that it is fuel-metered, or controlled using only the amount of fuel injected during the intake cycle.
Air/Fuel Ratio and EGT
Because the diesel engine's combustion event is so much slower, a great deal of its fuel ends up going unburned and going out of the exhaust pipe -- that's where the diesel's signature black smoke comes from. This isn't necessarily a bad thing, though, since that fuel helps to ferry heat out of the cylinder; but once that fuel reaches the exhaust, the heat and pressure in the exhaust stream creates an "afterburn effect," which spikes the EGT. A gas engine does exactly the opposite: Because oxygen is the gas engine's limiting reactant, extra oxygen in the cylinder (a lean mixture) will allow for a more complete combustion event, which raises the EGT. So, a rich mixtures raises the EGT in a diesel, and a lean mixture raises the EGT in a gas engine.
Exhaust backpressure is a major contributing factor to the EGT. High exhaust backpressure will allow gases to stack up inside the manifold and cylinder, trapping heat inside and leading to a domino effect of temperature increase as fuel exiting the cylinder afterburns in the manifold. Normal exhaust backpressure won't spike the EGTs by any significant amount, but adding a turbocharger will. The turbocharger acts like a cork in the system, especially under high-load conditions. If you've ever seen video footage of a turbo header glowing red- or white-hot on a dynamometer, then you've witnessed the effects of backpressure on the EGT. This is why a turbo header's tube material is usually two or three times as thick as a standard header's.
A diesel engine's exhaust manifold EGTs will typically run at about 300 to 500 degrees under no-load to part-throttle conditions, 800 to 900 degrees under a medium load and 1,000 to 1,200 degrees under a really heavy load and under full throttle. Temperatures measured at a point past the turbo will typically run 100-plus degrees cooler, depending upon the turbo rpm and flow. A normal gas engine will run about the same as a diesel under light-to-medium load conditions, but will average about 500 degrees under most conditions. However, the EGTs can easily surpass 1,500 degrees in turbocharged and performance applications.
A gas engine will typically maintain more stable EGTs than a diesel, owing to the fact that the gas engine's computer keeps air/fuel ratios fairly constant (which it does, incidentally, by using an oxygen sensor to monitor exhaust gas temperatures). Cylinder pressure and exhaust backpressure are the main contributing factors where the gas engine EGTs are concerned; upping combustion power by increasing compression ratio or adding a turbo or supercharger will spike the EGTs, especially if the exhaust system isn't up to the task of moving the gases away.