Speed OR strength
What is power and what is torque? What’s the difference between the two and why do petrol engines generate more of the one and diesel engines more of the other?
While the average motorist focusses on cost of ownership, boot size and the number of seats, petrolheads obsess about a different set of numbers: power and torque.
When these petrolheads were still young and played with Top Trump cards, they shouted out the technical specs listed next to engine capacity and horsepower and probably paid scant attention to the vehicle’s width and weight.
The guy with a Ford Capri card (400 hp.) in his hand smiled like the Cheshire Cat while his friend with the Renault 5 card’s (70 hp.) heart sank into his shoes.
But what is power and what is torque? Before we get to the technical definition, let’s first chat about petrol and diesel engines.
Cream soda or Coke?
The first truly successful sparkignition internal combustion engine was built in 1876 by the German Nikolaus August Otto. Before that, there were attempts by inventors in France and Britain, but Otto’s engines were simply better.
Otto engines were originally powered by coal gas, but later models used various gaseous fuel products like ligroin, propane and benzine.
In a petrol engine a mixture of air and petrol vapour is ignited by a spark in the combustion chamber. The ensuing explosion moves a crankshaft, which in turn rotates the vehicle’s wheels. It’s a short explanation that ignores a number of important steps, but, basically, that’s what happens. Petrol engines are known for their excellent power delivery.
Diesel engines came later when Rudolf Diesel built his compression-ignition internal combustion engine in 1893. Diesel’s original engine burnt finely ground coal dust and later models were powered by, among other things, peanut oil.
The fuel used today to power diesel engines is actually semi-processed petrol, but it could just as well be refined cooking oil or any other “heavy” flammable liquid.
In a diesel engine a mixture of air and fuel is compressed in high pressure until it spontaneously ignites. What follows is identical to a petrol engine’s drivetrain system, but the diesel engine is known more for its torque than its power.
Diesel, the fuel, is therefore named after this kind of engine (and the engine after its inventor) while the petrol engine gets its name from its fuel – petroleum.
But what is power and what is torque?
Power is the amount of work an engine can perform during any given time and the energy that’s released in that period of time. One horsepower equals 146 W, and 1 W is equal to 1 Joule that’s used in one second.
One Joule is the energy that’s transferred to an object when the power of one Newton acts on that object in the direction of its motion through a distance of one metre (therefore 1 Newton metre, or Nm). One Newton is the force that’s necessary to accelerate 1 kg of mass at one m/s in the direction of the applied force. And that brings us to torque (Nm).
Torque is a measure of how much turning power an engine’s crankshaft can exert on a driveshaft. Torque is equal to the compression ratio, expulsion and stroke of an engine, but not the revs because maximum torque is reached before maximum revs.
Besides its unique kind of ignition, a diesel engine has a longer piston stroke that exerts more leverage on the crankshaft and that provides excellent torque. A petrol engine on the other hand has a shorter stroke with smaller parts that move faster – power is directly proportional to revs per minute (rpm) – and although a petrol engine generates less torque, it’s more energetic, which is why it puts out more power (kW).
Think about it like this: A cheetah can cover a distance of 120 km in one hour (if it drinks Energade and pops Disprins), but after that it’s spent. A buffalo takes three times as long to >
cover the same distance, but you can ride on its back (if it allows you) while it’s doing it.
Buffalo or cheetah?
Look at the graphs of two engines – one petrol, one diesel – with a similar engine displacement. The reading was taken over a period of 10 seconds and it kicks off as soon as you floor the accelerator.
The petrol engine has a longer, more linear performance curve that will gradually climb from low to the maximum revs. At 8 000 rpm it reaches the maximum power delivery of 500 kW and after that the efficiency drops. After 10 seconds the engine reached an impressive 8 500 rpm.
The diesel engine’s max power delivery is lower – 450 kW – and it occurs at 4 500 rpm. After 10 seconds the diesel engine could only reach 5 750 rpm.
The petrol engine also has a longer, linear torque curve that gradually climbs from low revs to the max, and at 7 500 rpm the torque is maxed at 350 Nm.
The diesel engine reaches an impressive 580 Nm torque at just 4 250 rpm.
What does it mean for my vehicle?
With a diesel engine you have loads of torque at low revs, which comes in handy when you have to drive slowly over huge boulders. You only have to accelerate slightly to keep momentum. A petrol engine struggles with this and you have to keep the revs high, otherwise there’s not enough torque to conquer those obstacles and the engine might stall.
In sand the situation is reversed. If you’re driving through the Namib’s dunes, the even performance curve of a petrol engine will count in your favour. The flatter torque curve also prevents your tyres from digging in and the extra power helps to get you over those mountainous dunes. Diesels will struggle here because the diesel engine is generally not as eager and willing as a petrol engine.
If engines had star signs, petrol would be a fiery Scorpio and a diesel the dependable Taurus. An irate petrol scorpion will burst through walls, sending bricks flying, while the calm diesel bull will push over the wall and then drag it behind him.
A diesel engine converts 40% of the potential energy in its fuel to kinetic energy (to power your vehicle) and petrol engines convert only 25% into movement.
The rest of the energy is lost in things like noise and heat. You therefore save fuel with a diesel engine and a diesel engine doesn’t heat up nearly as much as a petrol engine – great news if you have half of Marienfluss’ grass between a hot exhaust and the chassis.
Torque, not power, is ultimately what will pull an off-roader out of the mud or have it climb up a steep hill. That’s why a diesel engine is generally the better option in 4x4 conditions.