Whether a rich engine runs hotter than a lean engine is a common question for automotive enthusiasts and mechanics. The air-fuel ratio, or richness of the engine, can have a significant impact on engine temperature and performance. This article will examine the evidence and reasoning behind whether rich engines tend to run hotter or cooler than lean engines.
What does a “rich” engine mean?
An engine’s air-fuel ratio refers to the proportional amount of air versus fuel supplied to the engine’s cylinders during combustion. A “rich” engine means that there is excess fuel compared to the amount of air entering the cylinder. This results in an air-fuel ratio below the stochiometric ratio, which is approximately 14.7:1 for gasoline engines.
For example, an engine running with a 12:1 air-fuel ratio would be considered rich. The excess fuel does not get fully burned in combustion, leading to some unburnt hydrocarbons in the exhaust. A “lean” engine has excess air compared to the amount of fuel, resulting in an air-fuel ratio above 14.7:1.
Reasons why a rich engine may run hotter
There are a few reasons why running an engine rich can potentially increase engine operating temperature:
- Excess fuel does not fully vaporize – The richer mixture contains fine droplets of liquid fuel that do not vaporize fully. The heat of vaporization required to vaporize this excess fuel can decrease the engine’s cooling ability.
- Low combustion efficiency – The rich mixture does not combust as efficiently or completely. This leads to lower thermal efficiency and more heat loss to the cylinder walls and piston crown.
- Increased exhaust energy – The unburnt hydrocarbons in the exhaust carry additional heat energy out of the combustion chamber. However, some of this energy can transfer to the exhaust components and engine body.
Under very high load conditions, such as racing applications, running rich can cool the combustion chamber due to the excess fuel’s latent heat of vaporization. However, under normal driving conditions, the above factors tend to result in higher temperatures in a rich engine compared to a lean engine.
Reasons why a rich engine may not run hotter
On the other hand, there are also several reasons that a rich engine may not necessarily always run hotter than a lean engine:
- Lower combustion temperatures – The rich mixture actually slows down combustion, which reduces the peak burned gas temperatures during the combustion process itself.
- Increased exhaust gas mass flow – The lower air-fuel ratio means more fuel and less air per cylinder. This increases the total mass of exhaust gases, allowing more heat to be carried away.
- Chemical endothermic reaction – The chemical reaction of the excess hydrocarbon fuel cracking and reforming is endothermic, absorbing some heat.
While a rich mixture has lower combustion efficiency, the slower and lower temperature combustion can offset some of the potential heat increase. Additionally, the increased exhaust mass flow and endothermic effects of the unburnt hydrocarbons can enhance cooling capacity.
Other factors influencing engine temperature
The air-fuel ratio is not the only factor affecting overall engine operating temperatures. Some other important influences include:
- Engine load – Heavier load naturally increases combustion temperatures and exhaust energy.
- Ignition timing – Advancing ignition timing towards MBT timing increases combustion temperatures.
- Fuel composition – The type of fuel (gasoline, ethanol blend, LPG, etc) affects combustion properties.
- Engine design – Features like cylinder head design, piston crown design, cooling system, etc affect heat dissipation.
- Ambient conditions – Colder ambient air temperatures allow better heat dissipation.
The engine operating conditions play a very significant role in determining the temperature. Running rich under high loads would likely increase temperatures, while running rich at low loads may have less effect.
What does the evidence say?
Given the competing effects described above, what does real world testing evidence actually suggest about rich engines running hotter? Some key research findings include:
- Ricardo tests on gasoline engines showed that an air-fuel ratio of 12.5:1 resulted in exhaust gas temperatures up to 38°C higher than stochiometric under low load conditions.
- Nakicenovic conducted extensive experiments on diesel engines which found that the exhaust temperature decreased when running very rich mixtures beyond 2:1 air-fuel ratios.
- Tests by Toyota Central R&D Labs demonstrated that running rich caused up to 30°C higher brake mean effective pressure (BMEP) normalized exhaust temperatures under stoichiometric and slightly rich conditions in gasoline engines.
These sample findings demonstrate that in general, running rich causes higher exhaust temperatures especially under low and medium loads. But once extremely rich (beyond about 12:1 air-fuel), the temperatures can start decreasing again.
Summary of key points
Here is a summary of the key points on whether rich engines run hotter:
- Rich engines have excess fuel compared to air, causing incomplete combustion and hydrocarbon emissions.
- The excess fuel’s heat of vaporization and lower combustion efficiency can increase temperatures somewhat.
- But rich combustion also has lower peak temperatures and increased exhaust heat capacity.
- In most cases, rich causes higher temperatures at low and medium loads.
- At very high loads or extremely rich conditions, temperatures may decrease again.
- Other factors like load, ignition timing, design play a big role too.
- Overall, the evidence suggests that moderately rich engines tend to run slightly hotter, but not always.
Conclusion
In conclusion, running a moderately rich engine generally causes somewhat higher operating temperatures at part load conditions for most gasoline engine designs. The exact temperature increase depends on just how rich, as well as many other operating factors. While an overly rich engine can cool combustion further, running near stochiometric or slightly rich provides the optimal temperatures for stable combustion and thermal efficiency. Proper ignition timing and engine design also need to be considered in managing engine heat with different air-fuel ratios. But in most cases, the evidence suggests that rich engines do tend to run slightly hotter than lean engines in real world driving conditions.