Motor Vehicle Maintenance & Repair Stack Exchange is a question and answer site for mechanics and DIY enthusiast owners of cars, trucks, and motorcycles. It only takes a minute to sign up.
Sign up to join this community
I am wondering why two car running at 100 km/h for example with a 100 hp consume less fuel that an engine of 300hp.
Even if I take a supercar lighter and more aerodynamics it will consume more.
But physicaly it require the same energy to drive constantly at 100 km/h, so does a more powerfull engine is less efficient ?
A couple of things to contemplate:
You need a certain amount of power at the wheels to maintain a certain speed.
This assertion is (correctly) made in the question.
At 100 km/h, a typical sedan would require roughly 10 hp at sea level to overcome the forces due to aerodynamic drag and maintain speed¹.
The fact that an engine can produce 100 hp or 300 hp peak power is of no consequence.
Who said that less powerful engines are more efficient?
They might seem like that, but Rory is right; the assumption that engines producing less power are more efficient isn't always true.
The concept of brake-specific fuel consumption (BSFC) helps us understand this, which is explained very nicely in this Hot Rod article.
Simply put, BSFC is just engineer-speak for how much fuel is consumed by the engine per unit of energy².
As the plot below shows, this number varies based on engine RPM. A smaller BSFC means better fuel economy.
Since each engine will have its own BSFC signature:
The BSFC curve of the high-power engine could be lower.
This will be especially true when comparing engines designed decades ago to modern-day marvels boasting things like EFI, variable-valve timing or forced induction.
Gearing.
If the BSFC curves are comparable, the gearing of the vehicle could mean that the RPM of the more powerful engine is in a more efficient part of the curve at a given speed.
A classic example of this is the "overdrive" gear found in some vehicles to help improve fuel economy at highway speeds.
¹ - The Bosch Automotive Handbook explains how to estimate this
² - Alternatively, BSFC = fuel flow rate per unit power
Your basic premise is not actually true. Often the more powerful motor will consume less fuel. Engines have different technological features that may affect this.
The reference 100bhp or 300bhp usually indicates the maximum power that engine can produce. So when you push the accelerator to the floor, the throttle is wide open, and you are producing a large amount of power.
At 100kmh, a high powered engine may be running really gently whereas a 100bhp engine may be struggling near its top end. I know that my 400bhp car consumes less fuel than my wife's 80bhp car at 90mph. But in accelerating to that speed, I will typically take 6 seconds and consume a huge amount of fuel, whereas she will take 25 seconds and probably not drop below 30 mpg. My car will still have a couple more gears and 100mph more, while hers will be in top gear at that point and near its max speed.
If I accelerated at the same speed as her, my car will use slightly more fuel than hers, as my 2.5l engine at low speeds definitely consumes more fuel than her 1.2l engine - that's a basic factor of size.
This sort of answers your question, despite having an opposite conclusion to your original premise.
A 100hp engine from 1900(year) would probably use a lot more fuel than a modern 300hp engine, so a lot depends on the design of the engine.
Comparing modern engines however, you are comparing two engines that are designed for different things. It is like asking why a shire (large) horse needs to eat more than a small pony when they are only carrying 1 small child each.
There are many factors, and some are already covered well above.
However on a typical petrol engine the amount of power required is controlled by the throttle. This strangles the flow of air into the engine, and this results in pumping losses.
If a car requires (say) 20hp for a constant speed then that is 20% of the available power of a 100hp car while only ~7% of the power of a 300hp car. Hence the more powerful engine needs throttling more with resulting greater pumping losses.
Things are not quite that simple as fuel mapping will vary with many factor, for example the mixture being richened up when accelerating. How much will vary greatly depending on the design requirements of that engine. 2 similar engines might richen the mixture up at different points in the throttle / rev range.
There are also weight issues. While weight has very little effect on the thrust needed to maintain a speed (it affects rolling resistance - a rule of thumb is that a car will require 1% of the weight in pounds as pounds of thrust to keep rolling) and essentially it doesn't change with speed (unlike thrust required to overcome aerodynamic drag which increases greatly as speed increases). Hence at a constant speed weight is of no great importance. However it makes a massive difference to the thrust required to accelerate at a reasonable rate. Double the weight and you need about double the thrust to accelerate at the same rate, let alone to accelerate more quickly (and after all, if you are choosing to buy a 300hp car rather than a 100hp car you probably do want to accelerate more quickly). In theory you will get back this loss by being able to coast for further, but in practice it is probably more likely the driver will brake and just lose the momentum.
Larger engined cars are often physically larger which can affect aerodynamics (which is frontal area x drag factor). A larger frontal area will increase the drag, but this might be countered to an extent if the larger car is longer giving more scope to design a more aerodynamic shape.
I am not an expert on the topic but i will try to answer in a laymans term.
Well first off all horse power is the amount of power produced by the car when fuel is burnt. There are many ways to increase the horse power. By increasing the amount of air flowing into the vehicle or by increasing the amount of fuel it is burning. Ok now lets us try to increase the horse power of our cars and lets see what effects it has.
Lets start cranking up horsepower of our car from 100 hp:
Now to increase the horsepower we may just get a better air flow system. Engine produce power by burning up fuel, the better they burn , more the power is generated. For fuel to burn better we need more oxygen, were do we get that, obviously increasing amount of air flowing in the engine(not too much though, we don't want a air heater).This blog explains various ways to do it. Using those methods you can increase up-to 50 horsepower. Now one thing here to notice that even you have now increased horsepower, it will also result in performance and a better fuel efficiency of the car.
So we increased our cars power from 100 to 150 and that also with much better performance and fuel efficiency.
Ok now lets try to increase it up-to 300
Now going on with better air flow will now not help us much so what we do. Remember last point engine generate power by burning fuel. So now to increase the power we have to increase the amount of fuel going in the engine. More means bigger blast, means more power.
Ok so why does it gives less mileage than the 100 hp one. Well cars with 300 hp(those are too many horses there, look out while driving) are generally designed to go at a higher speed. Generally these cars give better mileage at higher speed as compared at a lower speed, cause the have more fuel running in the piston compared that to 100hp car most of the fuel is not utilized correctly.
It is same like driving your 100 hp car in 2end gear, well you are capping the full potential of you vehicle no matter how aerodynamic it is, it's going to consume more fuel
Hope it helps.
It depends on vehicle weight, if it's a large and heavy car then counter intuitively bigger engine will get better mpg than a small one, because it won't have to rev as high just to move it and then at speed it will be able to sustain speed with much lower rpm than smaller engine would.
