Why do some large displacement engines create less power than smaller engines?



For example, why does 5.7L V8 Tundra Engine make 381HP/401 pound-feet and a 2.3L 4Cyl Focus RS make 350HP when other V8’s can make ~700HP.

In: Engineering

The engine in the focus is turbo charged, thus forcing more air into the engine. With more air requires more fuel. More of both creates more power. Where the v8 in the tundra I believe is naturally aspirated, so the only air getting into the engine is being sucked from the engine itself without any assistance from a turbo or a supercharger.

The short and simple thermodynamics answer is that it is at a lower pressure as it sweeps out its volume. Energy is pressure times volume. Either that, or it completes cycles faster, as power is energy over time.

In real engines this gets a bit more complicated.

For one, there are losses in the engine – wasted energy such as friction which can play a role. A very wide engine will have more efficiency than a comparable-volume engine with a very long stroke.

On top of that, pressure varies a lot in an engine. It starts low as the engine sucks air in, then rises as the piston comes down, then spikes at ignition and drops as the piston rises. It then drops as the valve opens, and rises a bit as exhaust is forced out. Hotter fuel-air mixes raise the pressure more. The duration of the burn changes the pressure curve. The heat of the air changes it. When fuel injection and ignition take place change it. Combustion efficiency changes it. There are a lot of variables involved.

In the samples you’re talking about, mostly rotation speed.

To a *very* rough approximation, power scales with displacement and speed. There are lots of other things involved, like /u/TheJeeronian describes, but if you want to get more power out of an engine you can increase displacement or increase crankshaft speed. Turbo/supercharging is effectively a displacement increase (by compressing the air more, you can make a smaller displacement engine hold more air).

A 350HP 2.3L is going to be running at about twice the RPM (ish) of a 381HP 5.7L. A 700 HP V8 is really screaming. Something insane, like an F-1 race engine, is a low displacement V6 but it’s going about 15,000 RPM.

The *stress* on the engine parts scales really strong with speed, especially at high speeds. A big engine, like a 5.7L V8, can generate a lot of power without spinning that fast. This means many of the parts, though physically big, don’t have to be as proportionally strong or precise. This means less exotic materials, easier tolerances, higher durability, less issues with weird stuff like valve float, etc. This is why pickup engines happily go 250,000+ miles while dragsters and race cars throw a piston every couple of races.

Making big things go *really* fast is very difficult. Balance becomes a huge problem, bearings get ludicrously expensive, lubrication failure becomes instant catastrophic disassembly rather than giving you seconds/minutes to shut down, etc. Rocket engines turbopumps live at the bleeding edge of this spectrum…truck engines live at the far other end, with the only thing past them being large ship engines.