I get why they are diesel, but why are they all of the inline 6-cylinder configuration? Why not V6 or straight 8 or freakin huge inline 4-cylinders?
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Gasoline engines typically run at a higher RPM, and most do not have a turbocharger to push air into them through the restrictive intake valves. This is also why it’s common for gas engines to have four smaller valves per cylinder rather than the minimum two valves (one intake and one exhaust).
Since gasoline engines “can” run at a higher RPM, they do, and then they multiply the torque output by using a higher gear-down.
Diesel fuel burns more slowly than gasoline, so the way to get the most power from them is to have a lower engine RPM per wheel revolution, and to have a long stroke, compared to the bore.
A Ford 302 V8 has a roughly similar bore to stroke, while a diesel will will have a longer stroke compared to the bore. A diesel might cruise down the highway between 1800-2000 RM, where a gasoline engine can easily attain 4,000 RPM’s when accelerating.
On the compression stroke, a diesel does not yet have any fuel in the cylinder, and as the air is compressed, it heats up enough to ignite fuel. A diesel will need a high-pressure fuel pump to overcome the high compression in the diesel cylinder, typically 18,000 psi. However, since the fuel is injected at the top of the compression stroke, the fuel “auto ignites” from the heat, and no sparking system ins needed.
Gasoline is much thinner than diesel fuel, so running the fuel injectors at high pressure is difficult, therefore they inject the fuel-mist near the outside of the intake valve (partial vacuum on the intake stroke), and compress the fuel and air at the same time. This means they cannot use high compression, and a car typically might use 10:1, while a diesel might use 15:1
Gasoline burns more completely at a specific ratio (around 14:1 air/gas), but diesel will burn at a wide range of ratios. Air is all around us and free for the taking, so diesels typically use a little more air than the perfect air/fuel ratio to ensure that all of the fuel gets used.
These principles can also help gasoline engines. The V8 302 is fairly well known and analogous of most passenger car engines. However Ford made many of the 300 ci “truck” engine (inline 6-cyl), which had a longer stroke than the 302. It had better low-RPM torque, but it could not run to as high an RPM as the 302.
Even so, the Ford 300 still did not have as high a compression ratio as a diesel, or as under-square of a bore/stroke ratio as a diesel.
If our goal is to make a 300 cubic inch diesel that is as cheap as possible, wouldn’t a 4-cylinder cost less than a 300 ci 6-cylinder? Yes, but…A 4-cylinder 300 ci diesel would vibrate more than a 6, and as others have pointed out, a 6-cylinder inherently has a good primary and secondary harmonic vibration profile, without needing to add a balance shaft (as others here have pointed out).
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