Our mechanical engineer’s thematic Q&A article this month is about automotive lubrication and the science of pumping petrol.
Our mechanical engineer’s thematic Q&A article this month is about automotive lubrication and the science of pumping petrol.
"NO CAR MANUFACTURER WILL RECOMMEND AFTERMARKET ADDITIVES FOR ENGINE OIL, AND NEITHER WILL OIL COMPANIES."
THERE is a wide range of engine oil additives available on the market. All claim to improve performance, fuel economy and engine life. However, there does not seem to be any that are endorsed by car manufacturers. On this basis, which would be the best engine oil additive for a specific car model?
Engine oils are manufactured to certain minimum standards set by the automotive industry. The specification includes several additives.
These additives are specially formulated by oil companies to enhance the oil’s ability to reduce friction, function as a heat-transfer medium (internal cooling) and inhibit corrosion.
Because of the extreme pressure and heat that oil is exposed to inside the engine, it must also be able to endure the harsh conditions without deteriorating too quickly.
Manufacturers run extensive tests when developing a new engine to ensure it meets the lubrication requirements for various alloys inside the engine.
It is, therefore, not surprising that no car manufacturer will recommend aftermarket additives for engine oil. For that matter, neither will oil companies. So, the best thing you can do for your car’s engine is to invest in a quality oil that meets the vehicle’s specifications, which are spelt out in the owner’s manual.
If fuel pumps in cars are located inside the fuel tank, isn’t there a major risk of fires originating from the tank?
In-tank electric fuel pumps are the most common types used today. The petrol is pumped from the tank to the engine compartment where it is fed to the fuel injection rail or, in some systems, to a highpressure pump first. Fuel not used is returned to the tank.
The reason there is no risk of fire or explosion in the tank is that petrol by itself will not combust. A certain minimum quantity of oxygen (and heat) is necessary and this is not present in the fuel pump.
In the early days when you could “flood” the engine with fuel by continuously pumping the accelerator pedal, the engine simply refused to start because the ratio of fuel to oxygen exceeded the required combustible mix.
In the absence of oxygen, even if you immersed a redhot piece of steel in a bowl of petrol, there will be no explosion or fire. Instead, the red-hot glow will be extinguished. Do not try this at home, though, as you cannot simulate an oxygen-free environment.
Do I need to change the oil in an automatic gearbox? If so, how often?
There are essentially no service routines recommended for automatic transmissions.
Most car manufacturers claim that the transmission is “sealed for life”. However, like any oil, the quality of an automatic transmission fluid deteriorates with use.
For the automatic transmission to remain healthy, it would be wise to have the fluid replaced every 25,000 to 30,000 kilometres.
The fluid replacement job requires some special equipment, which usually only transmission specialists or authorised workshops would have.
Some people say that filling up the fuel tank to its full capacity is detrimental to fuel economy. They suggest filling up just half the tank at every fuel stop. However, none of the motorists who believe in this idea has been able to explain why this is so.
Is it because a full tank causes more fuel to flow or more fuel to evaporate? Or is it just a motoring myth?
A fuel tank filled to the brim will not increase the fuel flow. The quantity of fuel consumed by the engine is controlled by the injection system, which is, in many ways, directly determined by your right foot.
The evaporation theory is also flawed because petrol lost to the atmosphere is, firstly, insignificant and, secondly, not a function of the volume in the tank.
Any change in fuel consumption that you detect, because you now fill only to half tank as opposed to full tank, can only be the result of weight difference.
The fuel tank capacity of an average family saloon is roughly 60 litres, which in terms of weight is about 50kg.
Hence, filling to half tank reduces the load the car carries by about 25kg.
This might make a small difference in the fuel consumption, but nothing that can compare with efficient driving habits.
