suzuki alto overheating

How does the lean mixture in a gasoline engine lead to overheating?

Great question! Let’s start by defining “lean mixture”. A “lean mixture” means one in which the volume of air is disproportionately high, while that of fuel is disproportionately low. ‘Disproportionate’ in terms of the stoichiometric air-fuel ratio - which depends upon the type of fuel used. For instance, the stoichiometric AFR for a petrol engine is 14.7 units of air to 1 unit of petrol. Now, if a charge contains 16 parts air for 1 part of petrol it can be considered a “lean mixture” for a petrol engine. On the other hand, if it’s, say, 11 to 1 in favour of air it’s a “rich mixture”. All engines have a primary cooling system, with more often than not, a secondary and a tertiary one too. For example - almost all cars today are water cooled, some motorcycles and scooters are air-cooled, while the rest employ a more efficient form of air cooling called ‘oil cooling’. On four-stroke engines, the motor oil serves as the secondary cooling system by absorbing some amount of engine heat as it moves through its interiors. Fuel is the tertiary cooling system. As any fuel vaporises, it takes some heat from its surroundings with it. If you pour a few drops of petrol on the palm of your hand and hold it for some time, you’ll feel a cooling sensation on the palm. Petrol does the exact same thing inside the engine too. Bear in mind, an engine is designed and manufactured according to its deemed usage and power output. The material used in them, the primary cooling system, service intervals and activities etc. are all chosen accordingly. A fast-revving, high-horsepower, water-cooled, KTM engine and a low-revving, stump-pulling heart of a Harley Davidson are like chalk and cheese in terms of nature, usage and constitution. Now, what you’ve been waiting for - the answer! The more the oxygen (a product of a higher volume of air), the bigger the combustion. “Bigger the combustion”. Now, isn’t that a good thing? Nope! As with everything in life, balance is the key to the survival and efficiency of your internal combustion engine. The bigger the combustion, the more the production of heat. Given that resources used for engine cooling are limited and can’t be changed suddenly, there’s an excess of heat. This surplus heat starts to degrade the motor oil thereby robbing it of its heat-absorption capacity. It may also ignite the fuel faster or earlier (or both) thereby reducing it’s cooling properties. This leads to a vicious circle of compounding heat, ergo- overheating. While on the one hand without the correct volume of air the combustion will be incomplete leading to a loss of power, with too much air (and too little fuel by proportion) combustion efficiency will lead to ‘unhealthy’ levels of heat production. The fact is - power and heat production in an engine are directly proportional. This explains why a Formula-1 car engine only lasts a few races, while that on a Maruti Suzuki Alto lasts years.