To discuss how a diesel engine offers increased torque over a petrol engine of a similar size, let's
quickly nail down what torque actually is.
Torque the Torque
We don't want to spend too much time trying to define torque; it's not a particularly simple concept so
there are no simple definitions, but in an internal combustion engine torque is usually defined as the
rotational force, or the 'turning power'. A good way to try to imagine the effect of torque is by
visualising a spanner turning a bolt. The more weight applied along the length of the spanner, the
easier the bolt turns. Extending the analogy to a car, imagine a huge, figurative spanner attached to
the wheel. The more force applied to that imaginary spanner, the easier and more forcefully the wheel
turns.
The measurement of torque works along these lines, and is usually recorded in pounds-feet. In the
spanner and bolt example, the torque is the force applied to the spanner, multiplied by its distance
from the bolt. Applying 3 pounds of weight to the spanner at a distance of 2ft from the bolt is the
same as applying 1 pound of weight at 6ft from the bolt, or 6 pounds of weight at 1 foot from the bolt.
As such, the torque in this example can be measured as 6lbs-ft of torque.
This is all pretty dry, technical stuff, of which the normal car owner might not really need expert
knowledge. It suffices to know that when it comes to internal combustion engines, the higher the amount
of torque, the more power there is available.
Diesels Develop More Torque
Diesels develop more torque compared with petrol engines of the same displacement. To illustrate this,
simply look at two similarly-specced ford Focuses. The diesel-powered 1.6 TDCi offers 158lb ft of torque,
and the petrol-powered 1.6i Vetec offers just 106lb ft. The diesel Focus produces 50% more torque than
the petrol Focus.
This yawning gulf in performance is not restricted to Ford engines. Because of the way diesel engines
work, and the fuel itself, there is more torque to be had. Large trains, submarines, boats, trucks - in
fact any mechanical device which requires significantly more power than paltry petrol can produce almost
unanimously relies on diesel. The car is almost the only internally combusting machine which hasn't
fully adopted diesel. Now, times are changing, and people are beginning to recognise that diesel is the
superior way to power their machines.
Why is Torque Beneficial?
So what makes torque so appealing? Why is it beneficial? Quite simply, the purpose of a car's engine
is to turn the wheels. Wheels are turned easier if more torque is being applied to them. By definition
then; an engine which produces more torque and turns the wheels more powerfully is a better engine. And
it's the diesel engine which produces the most torque.
The more torque your car produces, the larger loads your car can pull, and the better your car will
perform with heavy attachments or in difficult road conditions like mud, snow and water. If you live on
a hill it's handy to have a diesel sat in your driveway; a diesel car will have no problems pulling its
own weight up the hill.
Why do Diesels Produce so Much Torque?
There is no single reason why diesel engines produce so much more torque than petrol or any other
fuel; it's a combination of factors. Here's an outline of a few of the major contributors to torque
generation.
- Stroke length - diesel engines uses longer strokes. The power of the engine is applied as far
from the centre of the crankshaft - the pivot, in effect - as possible. This reduces the overall
RPM and makes the stroke slower, but means that the energy is loaded onto the crank in a much more
efficient way and produces greater torque.
- More energy in diesel - there is around 11% more potential energy in diesel than there is in
petrol. This means that diesel is converted into more power compared with petrol. Therefore
torque, horsepower, mileage, fuel efficiency, emissions and a number of other factors are all
vastly improved.
- Turbocharger boost - to assist in the ignition of the fuel in the cylinder, turbochargers are
often used to optimise the available oxygen and fuel mixture. The turbo boosts the torque production
as a byproduct.
- Effective cylinder pressure - in a petrol engine, the fuel is mixed with the air outside the
cylinder, which means that the combustion is very brief when it occurs. A diesel engine mixes the
air with the fuel inside the cylinder. This means that the combustion can last longer, forcing the
cylinder up and the energy onto the crankshaft with more power, and thus more torque.
The more you understand about how diesel engines work and how they deliver power to the wheels, the
more impressive the technology seems and the more astounding it becomes that petrol hasn't yet been
abandoned. Diesel engines are superior in virtually every practical way, and the generation of more than
50% more torque over petrol engines is a prime way to demonstrate it.
To summarise this information, diesel engines produce abundant amounts of torque compared with petrol
engines, thanks to the natural characteristics of diesel engine's performance.