If you're going to use an engine to power a car, truck, or motorhome you end up converting the engine's power into a force of propulsion. This force is used for acceleration, for cruising, for climbing grades, etc.
And, you can calculate how much force it'll take to accelerate a given mass to a certain speed within the distance you choose,....level ground or inclination.
Human beings are intelligent enough that they've designed different engines for different applications. We even say things like, "you wouldn't take a knife to a gunfight..."
Why are diesel engines better for certain applications? Here's a respected explanation, by a respected expert, Gale Banks:
Now let's take a look at diesel engines. The most significant differences from spark ignition engines are that diesels have no air throttle, fuel is injected directly into the combustion chamber, diesel fuel is used instead of gasoline, and the heat generated by the compression of the air in the cylinders ignites the fuel when it is injected. Diesels also operate in a narrower RPM band than gasoline engines, and have lower peak RPM. This is because with direct fuel injection it is difficult to have good air/fuel mixture formation and full combustion over a broad RPM range. The higher compression ratio and cylinder pressure of diesel engines also requires heavier rotating and reciprocating components that limit maximum RPM. Few diesels operate at speeds above 4000 RPM, and most run notably slower than 4000 RPM. Diesels are thermally more efficient than gasoline engines because they operate at higher compression ratios than gasoline engines, and they have fewer pumping losses. Additionally, diesel fuel typically has about 11 percent more energy per gallon than gasoline. Typical diesel fuel has 138,700 BTU per gallon whereas typical gasoline has 124,800 BTU per gallon. Diesels can also run on much leaner air/fuel mixtures than gasoline engines, especially at idle. Let's look at these things more closely.
Higher compression ratios result in more efficient burning of fuel in four-cycle engines. Most of today's gasoline engines operate with static compression ratios between 8:1 to 10:1. Special high-octane gasoline permits slightly higher compression ratios (usually for racing), but the added cost and availability of high-octane (high lead) fuels precludes the use of higher compression ratios in most applications. Diesel engines, by comparison, operate at compression ratios between 14:1 and 24:1. Gasoline engines cannot operate at these higher compression ratios because the air and fuel is already mixed together before it enters the cylinder. At compression ratios above those usually used in gasoline engines, the heat of compression would cause the mixture to self-ignite too early, causing "engine knock", parts damage, and a significant reduction in power. In a diesel, the air and fuel are handled separately. Only air goes into the cylinder on the intake stroke, and the pressurized fuel is injected after the air is compressed. During the compression stroke, the temperature of the intake air rises to as much as 1400º F. When injected into this hot environment, the fuel self-ignites. The injection of fuel is timed for minimal emissions, efficiency, and maximum torque generation. As with the spark timing in gasoline engines, the fuel injection timing is advanced slightly in some, but not all, diesel engines as the RPM rises. However, it should be noted that turbocharging quickens the burn time of the air and fuel as boost pressure rises. This means initiation of the fuel injection pulse must be retarded, or delayed, as boost pressure rises in a turbo diesel. In most diesels, the fuel injection timing changes for rising RPM and rising boost cancel each other out, and the timing of the beginning of the main fuel injection pulse remains relatively fixed. Depending on the design of the diesel engine, its intended usage, and emissions requirements, the fuel injection pulse can be set to begin as much as 24º - 26º before TDC, or it may be set to occur as late as TDC. In the Banks Sidewinder, for example, fuel is injected beginning 24º before TDC on the compression stroke.
Gasoline engines operate within a narrow air/fuel ratio range of approximately 12:1 to 15:1, although some modern "lean-burn" technology engines have been able to achieve significantly leaner air/fuel ratios.Diesels can operate with a broader range as rich as 15:1 or as lean as 60:1, however, going richer than about 22:1 to 25:l produces excessive temperature, soot, smoke, and poor fuel economy. Some aftermarket diesel chip manufactures simply dump in excessive fuel for power, causing the engine to operate in the undesirable rich range, as evidenced by plumes of black smoke. Thermal efficiency of diesels can be, and is, further enhanced with turbocharging to increase the available air (oxygen) to support combustion of more fuel. Gasoline engines cannot tolerate significantly higher cylinder pressure from turbocharging without creating preignition and/or detonation unless high-octane or ultra-high-octane gasoline is used.
Sidewinder engine on the dyno
Diesel engines also have significantly lower pumping losses than do gasoline engines. Since diesel engine power output and engine speed are regulated by varying the air/fuel ratio through controlling the amount of fuel injected, there is no need for an air throttle. Consequently, there are only minimal pumping losses on the intake stroke. This also means that unfortunately there is virtually no engine braking when the driver backs off the fuel throttle pedal unless an exhaust brake, such as the Banks Brake is installed, but that's a separate story. If the diesel is turbocharged, boost pressure can actually help generate "positive torque" on the intake stroke. Diesels do have high pumping losses on the compression stroke due to high compression ratios, but that is offset by nearly equal rebound on the power stroke, as explained earlier. Significant pumping losses occur only on the exhaust cycle, especially under boost conditions when a turbocharger is used, and such losses are more than offset by the added torque generated on the power stroke if the turbocharger is properly matched to the engine. The foregoing is assuming the exhaust conduits are sufficiently unrestricted for the size and power output of the diesel. If not, increasing the efficiency of the exhaust system will result in significant power gains. This is why products such as Banks Power Elbow, Monster Exhaust, Monster turbine outlet pipe, and Dynaflow mufflers work so well.
With all of the above taken into consideration, especially the lower pumping losses and greater thermal efficiency, it becomes clear why diesel engines are more efficient and produce notably more torque than similar displacement spark ignition engines. However, there's more to the story. In describing the operation of the gasoline engine, it was noted that once the intake valve had closed, the thermal potential for that intake charge was set. The same is not true for diesel engines. The thermal potential for the power stroke in the diesel can be controlled by the amount of fuel injected and the length of time it is injected. The only limits to that are the amount of air in the cylinder to support combustion and the capacity of the fuel injection system. Because fuel can be injected longer into the power cycle to sustain effective cylinder pressure, torque output can be dramatically increased. This is largely responsible for the massive torque numbers diesels are able to produce.
Willie & Betty Sue
Miko & Sparky
2003 41 ft Dutch Star Diesel Pusher/Spartan
Floorplan 4010
Blazer toad & Ranger bassboat