Eli5: 2 cars have the same Horsepower but different engine in terms of displacement (2.0L TDI vs 3.0L TDI) but what difference does it make and which one is better?
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It all depends on when the engines make their power. While both may have the same peak horsepower (this is how hp is measured) they may have completely different toeque curves, which matters more for drivability.
Now since you mentioned Diesel, the differences are much less important than gasoline. This is because Diesel engines tolerate much higher compression ratios than gasoline. Diesal engines also perform MUCH better under force induction than natural aspiration.
So to keep it ELI5 the simplest explanation is for diesal the engine displacement does not matter. As long as they are rated at the same they ought to consume the same amount of fuel and drive similar.
All things being equal if everyone drove at wide open throttle everywhere the engines would consume the same amount of fuel and produce the same amount of power.
All things arent equal and i dont know about you but as a professional uber driver i dont drive everywhere at wide open throttle. There are two schools of thought and I personally do not have a strong feeling either way. There are those who believe a big engine working not very hard will consume less fuel. I dont personally put much stock in that because most people speed and drive faster than engine speeds are designed for optimal fuel usage.
The other school would say more displacement more fuel consumption. This is because you can drive a small engine at low load its a smaller engine by design it requires less fuel to turn over. Less internal mass, less weight etc. In my personal experience the smaller the engine the less fuel it consumes !!!assuming part throttle driving!!! So in terms of fuel consumption either you believe that an engine with less load consumes less fuel or an engine thats smaller requires less fuel. I have not seen compelling evidence either way, but I HAVE NOT LOOKED IT UP. This is just based on my own mechanical knowledge.
If fuel consumptions not an issue for you there is no practicle difference. Some people – people who speed & drive on 2 lane highways a lot – will more appreciate the bigger engine because they “tend” to have more torque and do not have to rev as hard. Less relevant for Diesal because of the way it works. If I have a choice I will always go with a smaller engine because thats what i prefer.
Small engines being more efficent is also not the defacto case. My Imprezza had a 2l but it was so highly tuned and geared that on the highway it drank gas faster than my brothers 5.7l truck.
Imo gearing is more important than engine size. If you are interested for actual car selection drive both and do a lot of different types of driving. Not the little 4 turn fiasco dealers will take you on. Go on the freeway drive around town etc.
So which is better? For my money it is up to personal opinion. Smaller engines tend to be less popular with “normal” people because they value feel over fuel consumption. Speaking for North Americans anyway since fuel traditionally has been mega cheap. I do not like being shackled to something that costs me $100 twice a week to keep running, but i am a bit of a miser also i drive like 50,000km a year.
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