EGTs are a major concern because a diesel engine may begin to run excessively hot under a sustained load, i.e. towing up a hill, hard acceleration, etc. The problem is often more prominent on mechanically controlled diesels, which lack any failsafe or means of interrupting fuel flow in the event an engine begins to overheat. Electronically controlled diesels are able to pull fuel regardless of driver input, engine speed, and load if a problem is detected. EGTs are of the greatest concern in heavily modified diesel engines, especially those which feature components or devices that significantly richen the air-to-fuel ratio.
Air Fuel Ratios in Diesel Engines
The stoichiometric air-to-fuel ratio for the combustion of diesel fuel is 14.6 : 1. At this ratio, all fuel present will theoretically react with all air present and combusts completely. However, a diesel engine is unique from other internal combustion cycles in that it can operate within a wide spectrum of A/F ratios without adverse complications (spark plugs fowling in gas engines running rich, for example). A diesel engine will tend to run relatively cool when the A/F ratio is lean, or greater than stoichiometric. For a lean condition, there is an excessive amount of air available, although in theory all the available fuel is combusted. On the contrary, a diesel engine will have a tendency to run relatively hot when the A/F ratio is rich, or less than stoichiometric. During a rich condition, there is an excessive amount of fuel available, and therefore not enough oxygen to combust all the fuel completely. As a result, running rich results in the emittance of soot (black smoke) from the tailpipe. The richer the A/F ratio, the more smoke created and the hotter the exhaust gas temperature (EGT) becomes.
In practice, a diesel engine cannot completely burn fuel at a stoichiometric A/F ratio. This phenomenon stems from the fact that diesel and air are not ignited as a homogeneous mixture in the combustion chamber - the air and fuel only mix after fuel has been injected and nearly instantaneously auto ignites. Thus, there is simply not enough time for fuel and air to mix and combust entirely at a stoichiometric A/F ratio in a compression ignition engine. Since unburnt and/or partially burned fuel is essentially wasted engine, diesel engines typical operate lean of the stoichiometric ratio.
With regards to exhaust gas temperature, the richer the A/F mixture, the higher the EGT and the greater the amount of black smoke (soot) produced. Smoke has a tendency to appear as a light haze in the 16-18: 1 A/F range, but EGTs tend to be manageable under these conditions. However, as the A/F drops and the smoke begins thicker, EGTs can spike to dangerous levels.
EGT - How Hot is Too Hot
The absolute maximum safe EGT range is debatable. Our universal rule of thumb, regardless of engine make/model/year, is to not exceed 1250° F and not to operate in the 1200° F - 1250° F range for an extended period of time. This is a relatively conservative view, however 1) engines and engine components are expensive to replace and 2) this is a very manageable range. In racing and sled pulling you'll find that many high performance engines will experience exhaust gas temperatures well beyond this boundary, but it's important to remember that these engines are built for this sort of abuse. To justify this recommendation, it's important to understand several key factors that limit the resilience of an engine to high exhaust gas temps.
First and foremost, a high exhaust gas temperature is the result of a high combustion temperature and an inefficient burn - if the temperature of the exhaust gas is 1200° F, the combustion temperature is significantly higher. With that in mind, factory pistons are widely produced of an aluminum alloy. Aluminum, in its elemental form, has melting point of roughly 1,200° F. Fortunately, heat flow into the piston is impeded by the fact that combustion happens rapidly and the piston is constantly being cooled. However, the higher the temperature subjected to the face of the piston and the longer this heat has time to transfer, the greater the risk that the material will begin to yield.
Iron and steel alloys, such as those found in turbochargers, are more resilient to heat and absorb heat at a much slower rate. However, a turbine can spin in excess of 100,000 rpm, creating a tremendous centrifugal force on the rotating turbine wheel. As exhaust gas temperatures increase, the probability of failure grows exponentially. This is in addition to the fact that oil can begin cooking in the turbocharger bearings. For these reasons, it's important to consider the effects of EGTs on a turbocharger and the strain it places on its components. Additionally, cylinder heads today can be made of aluminum or cast iron. Subjected to continuously high temperatures poses the risk of head gasket failures. As the material absorbs heat, the cylinder head may begin to warp and/or the yield stress of the head bolts can be significantly reduced.
Because of these factors, in addition to the high cost of replacement or repair, we prefer to err on the side of caution with regard to EGT management.
Pyrometer basics
A pyrometer is a device that reads and displays exhaust gas temperature. A typical pyrometer includes the pyrometer, or pyro itself, a thermocouple, and a calibrated wiring circuit. A thermocouple is essentially a temperature sensor that relies on the principle of the voltage created by dissimilar metals in contact. Two dissimilar metals in contact will produce a small voltage proportional to the temperature of the two metals. Therefore, the pyrometer gauge reads the voltage (on the order of millivolts) across the thermocouple. In a pyrometer setup, the thermocouple is typically referred to simply as the "probe". The probe is installed in the exhaust system, often in an exhaust manifold or turbocharger up-pipe. A pyrometer nearly instantaneously relays the current EGT to the driver. Constantly monitoring the pyrometer is of the utmost importance in modified diesel engines, and is a good practice in even stock applications.
The pyrometer probe should be installed in either an exhaust manifold or the turbocharger up-pipe. It is not ideal to install a pyrometer probe at the turbocharger downpipe, as the exhaust gas temperature at the outlet of the turbocharger is always less than, and in some instances significantly less than, the temperature at the turbocharger inlet. This is do to the fact that a turbocharger is a waste energy recover device - as the turbocharger converts the engine's waste heat on the turbine side into pressurized air on the compressor side energy is extracted from the incoming exhaust stream, thereby lowering its temperature at the outlet due to the conversion of energy across the turbine. It is therefore only beneficial to know the temperature at the inlet of the turbocharger.
EGT Management
Excessively high exhaust gas temperatures are the result of a rich A/F ratio and are typically more common on modified engines. This does not imply that a factory engine should be able to pull a grade at full load without experiencing a high EGT condition, thus supporting the fact that even a stock vehicle can benefit from the installation of a pyrometer. Engines tend to encounter EGT problems when significant modifications have been performed to a fuel system without appropriate supporting modifications for the air management system. Large injectors, modified high pressure fuel pumps, and aggressive tuning will contribute to high exhaust gas temps on factory turbo systems.
In order to reduce EGTs, consider the following airflow upgrades:
• Wastegate modifications to increase maximum boost pressure (yield on the side of caution).
• Turbocharger upgrades and/or replacing the turbo with an upgraded unit that meets greater airflow demands.
• Aftermarket air intake system.
• Water injection to reduce exhaust gas temps on-demand as necessary.
• Upgraded intercooler or installation of an intercooler on applications that do not have one.
• Free flowing exhaust system.