If one car consumes almost twice as much gas as the other one for driving at the same speed, where does the excess power go to?



So let’s assume we have two cars. One is a really eco-friendly family car sedan, and the other one is a lamborghini. Both drive at a speed of 100kmh (62mph), And lets say we load up the lighter of the cars with some weights, so that they weigh the same. So lambo would on average, consume more fuel.

My question is, where does this extra power vanish to? Since fuel makes an explosion and explosion drives the torque of the car, where does this explosion power vanish to in a lambo? Since it generates more explosion, but still drives at the same speed.

In: Engineering

All of the energy from a car’s engine becomes heat, eventually. Some of it just takes a detour to push the vehicle.

The difference between a lambo and a civic is going to be mainly in the engine itself. The tiny engine in the civic can’t produce nearly as much power, but also has way less stuff moving around in it and less friction. The lamborghini’s huge engine wastes a ton of energy.

Another difference is aerodynamics, but between the two cars you mentioned at that speed the engines themselves are going to be the main difference.

The energy from fuel goes several places: acceleration of the vehicle; wind resistance; heat; noise; friction; etc. Using less fuel depends on how much of each of these things is present. High-economy vehicles usually limit how much acceleration is available, focus on reducing drag and friction, and even recouping excess energy from braking (in electric/ hybrid vehicles).

Option one; the heat is transferred through the walls of the engine block and/or into the coolant.

Option two; the heat is retained in the gas and shoved out the exhaust.

The fuel released energy but the engine doesn’t capture all of it as mechanical energy

The big engine in the Lambo is going to have a lot more moving parts that are heavier so it’ll lose more energy there than the smaller engine. This ends up being pretty significant for big engines and results in the no load fuel consumption being significantly higher.

The eco car will also have a eco tires which are much stiffer and provide less rolling resistance for better fuel economy but also less traction which is bad for a performance car

The eco car will also have a turbo. Your engine is trying to expand the hot gas to extract the energy from it before it exhausts it but it can’t get it all. The turbo provides another chance to extract missed energy and let’s you use a smaller engine (less friction and weight) to get the same power.

When you look at turbocharged sports cars they end up getting decent fuel economy numbers. Lambos are all big naturally aspirated engines(no turbos) but something like a Mercedes AMG C63 with a 4L V8 putting out 500 HP can still do 27 mpg on the highway

Tweak the engine parameters for efficiency over power and swap the tires and drive gently and it’ll compare pretty well to most eco cars, probably 35ish

Yes heat, but also realize that gearing and engine displacement are something to compare too. The civic might have a 1.8L spinning at 2800rpm to go that speed and the Lambo has a 5.2L engine doing 1500rpm. So, 4stroke math, says the civic is displacing ~2500L in a minute, and the Lambo is displacing ~3900L a minute. If all else was the same, the Lambo would be burning ~56% more gas at this speed.
Engine HP is measured with rpm*torque/constant. The civic has maybe 4k more till redline, but also not much more torque, so it might only only be using 1/2 it’s total HP the engine can make.

Once cars are at a cruising speed only elevation changes, wind resistance and tire weight/resistance will dramatically change how much gas is used in driving a distance.
The Lambo would have 6k+ more rpm and probably another 50% torque….so it’s probably only using like 20% of the engine’s peak HP.

To twist everyone else around, it’s also horses for courses. Top Gear put a Prius and a powerful BMW M3 on a track. The Prius driver drove as fast as he could around the track, and the BMW driver was told to just keep up. In the end the BMW got better gas mileage.

Why? Because of lower power the Prius driver had to drive like a maniac, pedal to the metal, and the BMW driver just cruised because of his much higher power. He didn’t have to floor it, his engine stayed within efficient power bands.