Over here . .
Why do heavy vehicles almost always use diesel engines?
Someone wrote; "I can get umpteen Nm of torque from a motorcycle engine and a big gear ratio, yet they do not use them in heavy vehicles. So, torque alone is not the answer".
In response; yes, I can understand how you could come to that conclusion, as I love/ride bikes and also have one that is turbocharged that possesses lots of torque.
That said, simply put, torque production with high durability and low friction (the latter two mostly coming from low engine speeds and the use of heavy duty powertrain design approaches) is really the main reason diesel engines are used.
Extrapolating this and also your earlier response about motorcycles; if you look at Motorcycle engines of comparable size/cylinders to small car engines of similar capacity, you will see that the car manufacturers often still decide to adopt major design changes to their powertrain - rather than just using the same engine design approach.
So clearly there are different considerations and these come down to how the torque is manifested and delivered by various engine configurations and manufacturers.
These design changes are due to the fact that the car (and particularly truck) engine must produce more torque, and - if possible - more if it, down lower in the rev range; in order to provide the required thrust for all the (varying) weight the car itself always possesses and can carry.
Motorcycles, on the other hand do not have quite such a large potential for varying weight (as cars), and as such their engines don't need to be straddled with these same design limitations/specifications; hence their emphasis on high rotational speeds, light weight, high volumetric efficiencies, and KW - rather than (specifically) torque.
Additionally, motorcycles also are, by/large, (sold on them being) performance oriented machines, and in any event (particularly for the ones less than 1000cc) that means that they usually must rotate their crankshafts at reasonably high speeds in order to produce meaningful torque and power.
This means (amongst other considerations) motorcycle engine designs - unlike small engine passenger cars - don't have to compromise high crankshaft speeds for low down torque; as most car engines designed - as above stated - do - as those car engines will simply not rev comparably high even though the same capacity engine could otherwise (in a motorcycle ) easily be designed to.
So, we have an engine design trend for vehicles (that are designed to carry varying weight) that reads like this; more *constant/high value torque over a greater rev range, preferably starting as low as possible within the rev range, and with efficiency, reliability, and economy if possible.
Motorcycle engines fail at the first *specification and as such they can never do this for the reasons above, others, and also because Torque is a product not just of the combustion process and its resulting forces - but also because it is a product of the engines rotating/reciprocal weight; inertial torque.
And, motorcycles (particularly their engines' rotating components) are usually quite light - not in the least to achieve the high revs they need to produce.
Therefore a motorcycle engine/design not only fails to produce the meaningful values of (inertial and composite) torque where it is needed in order to perform the tasks of heavy vehicles - but the torque it does produce is largely combustion-force dependent, and as such (even with modern gearbox design approaches) is still too susceptible to changes in weight and vehicle ascension/gradient for the tasks required.
This design limitation and problem (associated with applying motorcycle engines to heavy vehicles) largely and most obviously manifests itself as a bore, stroke, reciprocal weight, and torque bandwidth issue.
Try riding a motorcycle around the city - especially if it is hilly - that has a passenger and/or (particularly a) motorcycle trailer attached to it, and you will see not only how impractical it is to grab 4K/rpm - 5K/rpm every time you want to take off even on a really powerful motorcycle - but also you will see how long your clutch lasts and ceases to be non-smelly.
Yet, (at best/least) the same weight displacement considerations are precisely what cars must accommodate all the time and reliably; not to mention trucks.
That all takes us back to my earlier comments about heavy vehicles, diesel engines, and torque; as they produce high torque values quite well, at low engine speeds, over a wide rev range, and they do it reasonably reliably too.
Aside from heat, noise, and exhaust; engines only ever produce Torque and Horsepower, and the latter is a function of the former.
Reliable and cost effective torque is the name of the game, and that's why diesels where invented and it's primarily why they're mostly used in heavy vehicles today.