eli5 why trucks don’t use small displacement high revving engines


Like, you see cars like ferrari making absurd power figures with small block engines, like a 4 liter making 800hp. Is it just a cost thing that they would rather use a 10 liter? It makes the same power, and thanks to the transmission it’ll get the same torque at the wheel.

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I should clarify, when I say trucks I’m talking those tractor trailer style, 18 wheelers as we call them in the usa

The higher the revs, the lower the lifetime and reliability of an engine. Trucks are generally large enough to fit large engines – so there isn’t any incentive to get light, small and high power. More important are high torque (to pull heavy loads) and high reliability (ie less maintenance dollars per mile).

High rev engines are difficult to maintain – truckers do not want to spend time in a service shop. Larger engines tend to be simpler and much quicker to repair at much less cost. Given that a truck will have to weigh many tons anyway, adding a few hundred pounds of engine isn’t a big deal.

They need an engine capable or pulling 40 tons up a slope without breaking down and a high revving engine won’t cut it.

It’s a matter of how much power you need at which rate for the load you’re trying to move. A big engine from a truck has loads of power, bit since it’s huge, it delivers it at a low rate. A small engine in a car has less power, but delivers it at a high rate. Both of these scenario’s end up with the same horsepower, but a different application. A car doesn’t weigh much (in comparison to a truck) so doesn’t need a lot of power, so a small engine will suffice. A truck weighs a lot, so it needs more power and thus a big engine.

The ferrari engine could well cost 10x what the truck engine does. It will also burn more fuel and wesr out fsr quicker due to needing to rev so high for the same power.

The truck engine likely also has a smooth flat power curve where it almost constantly outputs 800hp The ferrari by comparison maybe only puts out 800hp at a very narrow rpm range. So to use the ferrari engine for pulling you would need q gearbox with some ridiculous number of gears or a cvt made of admantium.

The engines they need require torque. A horsepower rating at high rpm is great, if you can get the vehicle to move and up to a speed that can maintain that rpm. You need torque to get stuff to move and the high HP small engine will not be able to pull it off the hop.

Another important component is high torque diesel and fuel economy. Nearly all large towing capacity tractor trailers use high displacement inline six cylinder diesel engines. Many engines exceeding 1500 lb/ft of torque. A comparable gas engine (longevity concerns aside) would have vastly worse fuel economy. And when you run an industry like trucking where fuel prices affect your profit margins, you can rest assured that engineering teams design the engines that go into these trucks help to maximize industry profits.

For fuel efficiency and fuel cost, you want a diesel. Diesel engines are low revs, due to their design. They are normally bigger with heavier moving parts which makes it harder to run at high speed. It also wear it out faster.

Acceleration of large vehicles is limited by torque, the maximum cruising speed is limited by power. There are practical limitations to how high you can make a gear ratio, 10:1 is about the upper end which means if you need more you either need an exotic transmission with a worm gear in it or an engine which generates more torque.

In an ideal world with lossless gears of infinite strength you could just set the gear ratio to whatever you want and only worry about engine power, but in the real world you need to keep your gear ratios relatively short to avoid having a massive transmission or just shredding gears as you go.

High rev engines also tend to be running at the upper limits of their materials and need more maintenance to keep them going. A big diesel is running well below the limits of the materials and the weight of the engine doesn’t matter when you have 20 tons of cargo in the back