How big is a semi truck engine?

It varies, some Over-the Road trucks have a modest number of cylinders ( like 6) but large volume capacity- over the summer I saw a l5 or l6 Liter Diesel on a Kenworth rated at 475 HP ( idle rev- the driver said- that means well over 500 horsepower underway!) there are even larger off-road trucks- also, last summer I saw a Komatsu Off-road dump truck ( on a trailer being towed to, presumably a road-building site) this Japanese-American monster has an eighteen cylinder (V-l8) Diesel engine and electric final drive, rather like a locomotive. Not to be outdone, Caterpillar markets a V-20 Truck that is rated at I believe 4000 HP. all of these are Diesel. Hope this helps. Woof Woof. ( Mack sign off).