Why People Use Diesel Engines
What differentiates diesel engines from other kinds of engines?
HIGHER COMPRESSION RATIO
“The compression ratio is higher, there is more power.”
Technically speaking, the compression ratio of an engine is the comparison of the total volume of the cylinder at the bottom of the piston’s stroke divided by the volume of the cylinder remaining at the top of the piston’s stroke. Since we are familiar with gasoline engines, let’s quickly discuss their compression ratios and a condition that spells disaster in a gasoline engine, detonation or “pinging.”
Serious damage to gasoline engines can result if you attempt to run a high compression ratio with low-octane fuel. Detonation or “pinging” is the ignition of the fuel due to the high temperature caused by a high compression ratio/high pressure developed by a given design. The fuel is ignited prior to the spark of the spark plug resulting in rapid uncontrolled burning.
Remember, the diesel is a “heat engine,” using heat developed from the compression of air. High compression ratios (ratios ranging from 14:1 up to 20:1) are possible since air only is compressed. The hot compressed air is sufficient to ignite the diesel fuel when it is finally injected near the top of the compression stroke. A high compression ratio equals a greater expansion of the gases following ignition and a higher percent of the fuel’s energy is converted into power! The diesel compression ratio is higher, there is more power!
THE INJECTION SYSTEM
“It’s in the injection system.”
Diesel designed the “heat engine” using the injection of fuel at the last moment to ignite the compressed air. Understanding the heart of the diesel, the fuel pump, is another key to answering the fuel-efficiency question.
THE GASOLINE ENGINE
A gasoline engine is stochiometric. Stochiometric: the quantitative relationship between two or more substances, especially in processes involving physical or chemical change. With a gasoline engine there is a stochiometric equation of 14 parts of air to one part of fuel. Remember, always 14:1; whether at idle or full throttle, the fuel and air are mixed outside the cylinders in a carburetor or injection manifold and the mixture is introduced via the intake valve. 14:1 always.
THE DIESEL ENGINE
Fuel and air in the diesel design are not premixed outside the cylinder. Air is taken into the cylinder through the intake valve and compressed to make heat. Diesel fuel is injected near the top of the piston’s stroke in an amount or ratio corresponding to the load on the engine. At idle, the air-to-fuel ratio can be as high as 85:1 or 100:1. At full load the diesel still boasts a miserly 25:1 or 30:1 ratio! It is in the injection system.
THE FUEL PUMP
One type is the Inline-style. Plunger pumps actuated by the pump camshaft send fuel pulses through six high-pressure fuel lines to the injectors. The pressure opens the injector valve allowing fuel to pass into the combustion chamber.
Metering of the fuel (for example, at idle 65:1 or full load 25:1) is controlled by a fuel rack and gears which rotate a metering helix to allow fuel into the six plunger pumps.
Another type is the rotary-style fuel pump. Think of this pump like a mini automobile spark distributor. A rotary “head” sends fuel pulses through the high-pressure fuel lines to the injectors. Just like the in-line fuel pump system, the pressure opens the injector valve and fuel is injected.
“The btu value of diesel is greater.”
Quite true, the btu, or British thermal unit, for diesel fuel is 130,000 btu’s per gallon and a weight of 7.4 lb/gal. The value for gasoline is 117,000 btu’s and a weight of 6.0 lb/gal. If we go back to our basic physics rules for energy, you’ll note the fuel in the tank has potential for work if it is injected into the cylinders and, combined with the compressed heated air, ignited. The piston is forced downward, the crankshaft rotates, the alternator turns.
The diesel design (the “heat engine”), compression ratios (the “heat engine”), the fuel injection system (allowing 85:1 down to 25:1 air-to-fuel versus gasoline at 14:1) and fuel btu’s (diesel fuel has more power), all these diesel attributes add up to more KW per gallon of fuel!
Final notes: We’ve covered the principle of diesel operation and the high compression ratios needed to make the heat for diesel engine combustion. The high compression ratio causes the engineers to design, test and manufacture the block, heads, head bolts, crankshaft, connecting rods, rod bolts, pistons, piston pins, etc., with greater structural capacity. In other words, diesels are inherently heavy in relation to their gasoline brothers.
WHY NOT USE GASOLINE?
Your generator could be fueled by gasoline, propane, or natural gas, but it really ought to be fueled by diesel.
- A diesel generator produces twice the power per gallon of fuel that a gas generator does.
- A gallon of (“off road”) diesel is cheaper than a gallon of gas.
- Diesel fuel does not blow up. In fact, it is hard to get it to burn at all.
- A diesel motor will last four times longer than a gasoline engine.
- Untreated diesel fuel lasts longer in storage than untreated gasoline.
- Treated diesel fuel lasts longer in storage than treated gasoline.
- Diesel fuel treatment costs less than gasoline treatment.
- Spoiled diesel can be reconditioned to refinery specifications. Spoiled gasoline can not.
- Unmodified diesel motors can be run on vegetable oil.
Natural gas should not normally be relied upon if you live in an earthquake prone area. The whole point of a generator is to be independent of, and replace something a utility failed to provide. Why would you want to hook up your generator to yet another utility?
The engine rpm’s, (and therefore the frequency output of the generator) will not be very stable if you’re running a lower torque engine with a lower energy fuel like propane/NG and you’re running that combination at full capacity. Frequency stability isn’t that important to a power saw but it is important to your electronics.