Why Diesels Make So Much Torque

For those used to gasoline engine torque numbers, diesel torque verges on the unbelievable.

The Banks Project Sidewinder, a Dodge Dakota pickup fitted with a modified Cummins 5.9L (359-cubic-inch) in-line six-cylinder diesel, made 1300 lb.-ft. of torque during its record-setting Bonneville Salt Flat runs at 222 MPH. That’s not a misprint – we’ll repeat, 1300 lb.-ft. of torque! That’s over 220 lb.-ft. per liter. Yes, this engine is turbocharged, but consider this: a good modified marine twin-turbo big-block (454-cubic-inch) Chevy only makes about 1000 lb.-ft. of torque, and a similarly modified 350-cubic-inch twin-turbo small-block will do well to make 775 lb.-ft. of torque. So, what’s going on here? Why does a diesel make so much torque compared to gas engines?

There are actually a number of reasons why diesels make so much torque, but the big reasons are stroke length, turbocharger boost, and average effective cylinder pressure. Turbo-diesels typically operate at higher turbocharger boost levels than do comparable gasoline engines. Production pickup and motorhome diesels routinely make 15 to 30 PSI peak boost, and it is not uncommon for a modified turbo-diesel to hit 30 to 50 PSI peak boost, and that definitely makes torque by reducing pumping losses on the intake stroke and increasing cylinder pressure on the power stroke. By comparison, 15 PSI boost in a gasoline engine is a lot of boost. Diesel fuel has about 11 percent more energy per gallon than gasoline too. And if all of that isn’t enough, a diesel is also more efficient than a gas engine. We could leave it at that, but the idea here is to dig a little deeper into these things to give you the knowledge to understand and build on for future discussions. With that in mind, let’s take a closer look at diesel stroke length and cylinder pressure.

The old adage that a long stroke is good for torque is true. The longer the stroke, the more offset the crankshaft pin has from the centerline of the crankshaft. This means the connecting rod can exert more leverage to turn the crank as the piston descends on the power stroke. The Merriam-Webster dictionary defines torque as: “a force that produces or tends to produce rotation or torsion…; a measure of the effectiveness of such a force that consists of the product of the force and the perpendicular distance from the centerline of action of the force to the axis of rotation.” That’s a mouthful, but the part about “the product of the force and the perpendicular distance from the centerline of action of the force to the axis of rotation” is critical to our understanding of generation of torque in an engine. The greater the crankpin offset to the centerline of the crank, the greater this perpendicular distance will be for any degree of crankshaft rotation after top dead center (TDC) and before bottom dead center (BDC). Consequently, the more leverage the pressure on the piston top that produces or tends to produce rotation of the crankshaft, or the more force it can exert: torque.

Diesels are usually designed as long-stroke engines specifically to generate torque. This is possible because the heavy components necessary to withstand the high compression ratio, and high cylinder pressure on the power stroke, of a diesel limit engine speed anyway, so excessive piston speed associated with long strokes isn’t a problem. Some heavy-duty truck diesels operate at a maximum of only 2200 RPM. Lighter-duty truck diesels may redline at 3000 to 3500 RPM, and 4000 RPM is considered a “high-speed” diesel. Let’s just summarize by saying that diesels are usually designed with as long a stroke as practical for the desired peak engine speed. For example, the 5.9L Cummins in Project Sidewinder has a stroke of 4.72 inches and a bore of only 4.02 inches. By contrast, most gasoline automotive engines have a stroke length that is shorter than the bore diameter.

There’s a negative side to increasing stroke other than being an RPM limiting factor. The longer the stroke, the greater distance the piston must move during each stroke. At a given RPM, that means the piston has to travel faster (higher average velocity) to cover that distance than it would if the stroke was shorter. Now remembering that the piston is essentially stopped at both TDC and BDC, to achieve that higher average velocity during the stroke, the piston must accelerate faster during the first half of the stroke and decelerate faster during the second half. This increased acceleration and deceleration takes energy – lots of it. Fortunately, the negative torque of accelerating the piston is largely balanced by the positive torque of the deceleration, but the loads on the crankshaft, piston, the piston pin, connecting rod, and rod bearing during all four strokes of a four-cycle engine increase dramatically with increases in stroke (or piston speed).

Now let’s discuss effective cylinder pressure on the power stroke of a diesel as compared to that in a gasoline engine. We’ve already mentioned that higher turbocharger boost raises the effective cylinder pressure, but let’s look at what else comes into play. In “Understanding Today’s Diesel” elsewhere on this site, the way fuel is introduced into the cylinder is thoroughly discussed for both gasoline and diesel engines. Gasoline engines mix the fuel with the air before it enters the cylinder, so when the intake valve closes, the power potential of that air and fuel charge is set. The timed spark ignites the mixture and cylinder pressure rises to a peak at roughly 15º after TDC. Because the combustion process takes time, combustion may or may not be complete by 15º after TDC depending on engine RPM, but for all practical purposes, we can say that the process of combustion is concluded early in the power stroke and that no more heating of the working fluid (the gases in the cylinder) occurs. This means the force acting on the piston top is highest at a time when the connecting rod has very little leverage on the crankshaft pin. As the crankshaft continues to rotate past TDC, the leverage the piston can exert increases, but the pressure on the piston top is dropping quickly. This, too, is discussed in the aforementioned article.

Once you envision when combustion occurs and the relationship between cylinder pressure and leverage on the crankshaft, it becomes obvious that if we could continue the burning process longer into the power stroke, additional cylinder pressure could be generated to push on the piston top as connecting rod-to-crankshaft angle improves for more leverage, and hence more torque. This is exactly what happens in a diesel. Because the fuel is injected into the cylinder after the intake valve is closed and the air is compressed, the length of the fuel injection pulse, called pulse width, can be extended well into the power stroke. This means the average effective cylinder pressure acting on the piston is higher in a diesel than in a comparably sized gasoline engine. The higher turbo boost pressure, high compression ratio, and greater heat content of the fuel all add to the generation of cylinder pressure that is substantially higher than in gasoline engines too, but it is this continued injection of fuel that really makes the big torque numbers for diesels. And all of this taken together makes it apparent why diesel engines have to be built with such robust parts to withstand this high cylinder pressure and torque.

Of course, injecting fuel beyond a certain point on the power stroke of a diesel does no good because there isn’t time for combustion to conclude before the exhaust valve opens near BDC.

The more you learn about diesels, the more impressive the engineering science behind them becomes. Diesel may well be the fuel of the future, and hot rodding turbo diesels may become as common as hopping up a small-block Chevy. We’re still in the early days of hot rodding diesels, especially modern diesels with electronic fuel management. Now is your chance to be one of the pioneers in this growing technology. Besides, it’s fun to kick gas with a diesel! Just ask Gale Banks.