[BobbyJo]

This is actually entirely untrue. Combustion engines have had something of a renaissance the last 15 years as new manufacturing and simulation technologies have enabled much more precise machining and smarter designs. This has allowed old designs to get much more efficient, and new designs once infeasible to become commonplace.

The power output of an engine that achieves 30mpg today is easily double that of just 15 years ago.

Turbos are a good example. Before, they were very expensive to add to a car, and altered the driving experience such that they weren't very practical for the average driver. Now, essentially every new combustion engine has them because you can lower displacement and actively manage power output to a greater degree, pushing up efficiency.

Some cool things are on the horizon as well, like camless engines and opposed piston diesel engines.

[xyzzyz]

> Now, essentially every new combustion engine has them because you can lower displacement and actively manage power output to a greater degree, pushing up efficiency.

In Europe, yes, due to extremely stringent fuel consumption regulations — essentially, the engine needs to have very small displacement, so you need to add turbo to get at least some power out of it. A typical new car in Europe has turbocharged 1.2L engine or something in this neighborhood. In the US, however, naturally aspirated 2.0L engines are still king.

[rsync]

"Turbos are a good example. Before, they were very expensive to add to a car, and altered the driving experience such that they weren't very practical for the average driver. Now, essentially every new combustion engine has them because you can lower displacement and actively manage power output to a greater degree, pushing up efficiency."

This is true. In fact, some manufacturers (Volvo ?) have new models that are both super and turbo charged.

But this isn't cost-free - these are tremendously complicated engines running at very close tolerances. These will have higher failure rates and will be more expensive to maintain and fix.

Remember, kids: There's no replacement for displacement.

[Gauge_Irrahphe]

The only major "improvement" has been the ability to cheat the tests and pass the regulations, because those who made them didn't realize you cannot dramatically improve such an old technology.

>The power output of an engine that achieves 30mpg today is easily double that of just 15 years ago.

That's a complete lie.

[throwaway0a5e]

>That's a complete lie.

And you're completely wrong. And this can be trivially easily confirmed by looking up the power outputs of vehicles that get any given fuel economy today vs the power outputs of vehicles of like fuel economy from decades past.

With respect to small diesels specifically, ever since diesels went from naturally aspirated indirect injection to direct injection and turbos power output has skyrocketed while fuel economy has mostly flat-lined.

Fuel economy is mostly dependent on the displacement of the engine and the load to do work (like cruise the highway or plow a field at part throttle) so it doesn't vary much unless the efficiency of converting fuel to force improves (hard) or the demands of the task decrease (hard for cars, even harder for industrial applications).

[Gauge_Irrahphe]

I actually looked it up. A late 19th century engine had a 26% efficiency. 1960's engines had ~34% efficiency and in the 2010's it was ~43%. So the same fuel consumption would give you 26HP in 1900, 34HP in 1960, and 43HP today.

[BobbyJo]

Source?

Edit: Also, you're confusing theoretical and practical efficiency. For instance, a V8 engine can be 30% efficient in absolute terms, but if you can use an I4 for the same application because you now have a Turbo, your real world efficiency is way higher. This is exactly what we see happening in performance cars an large trucks: big displacement being replaced with turbos and hybrid systems.

[Gauge_Irrahphe]

Wärtsilä 31, which claims to be the most efficient engine ever built, claims 167.7g/kWh.

Diesel's engine used 317g/kWh.

So the amount of fuel used has not even halved in more than a century.

[inetknght]

Fuel efficiency and horsepower are very different measures.