Diesel 101 (HIGHLY abbreviated)

by Tom

Diesel Theory
Brief History
Diesel injection
- Indirect-injection (IDI)
- Direct-injection (DI)
Two types of injectors
Turbo chargers
Why “Glow Plugs”?
Diesel fuel
Worlds most powerful diesel engine!

(Much of the information presented here has been gleaned from various sources, including online forums, help columns, etc., and is not all our own original material. This information has been reviewed, concentrated into digest form and should be to the point and easily understood, but if you have questions, it’s likely that others do to, so please ask for further clarification.)

Diesel Theory
There is not the time to go in-depth into diesel theory and operation, so we will discuss operation in the simplest of terms and leave any desire for in-depth knowledge to further discussion.

Brief History
Rudolf Diesel developed the idea for the diesel engine and obtained the German patent for it in 1892. His goal was to create an engine with high efficiency. Gasoline engines had been invented in 1876 and, especially at that time, were not very efficient.

The main differences between the gasoline engine and the diesel engine are:

• A gasoline engine intakes a mixture of gas and air, compresses it and ignites the mixture with a spark. A diesel engine takes in just air, compresses it and then injects fuel into the compressed air. The heat of the compressed air (at a substantially higher compression ratio than a gasoline engine) lights the fuel spontaneously once it is injected into the super-heated compressed air.
• A gasoline engine compresses at a ratio of 8:1 to 12:1, while a diesel engine compresses at a ratio of 14:1 to as high as 25:1. The higher compression ratio of the diesel engine leads to better efficiency.
• Gasoline engines generally use either carburetion, in which the air and fuel is mixed long before the air enters the cylinder, or port fuel injection, in which the fuel is injected just prior to the intake stroke (outside the cylinder). Diesel engines use direct fuel injection -- the diesel fuel is injected directly into the cylinder.

Diesel injection

Indirect-injection (IDI)
The fuel is forced through an injector at a moderate pressure and “sprays” into a pre-combustion chamber where the fuel begins the burning process. SVO/Biodiesel fuels work the best with this lower pressure, pre-chamber system. In a prechamber (or divided chamber) engine design like this, there is a relatively small volume separate from the main combustion chamber, but connected to it via one or more passages. Fuel is injected into this smaller chamber where it is ignited by compression heat. Called indirect injection, since the fuel is not introduced into the main combustion chamber above the piston.

Indirect injection usually allows for a lower overall fuel-to-air ratio and lower pollutant emissions, since it is easier to control the process for the smaller pre-chamber volume. The connecting passage and space can be designed to produce good swirling and mixing of the fuel and air, hence better combustion. The flame front is well established when it enters the center of the main chamber.

Direct-injection (DI)
The fuel is injected at extremely high pressure directly into the cylinder above the piston and not into a combustion pre-chamber. It is worth noting that direct injection engines are generally more economical and since the injection can be more precisely controlled, they can be made quieter (the older DI engines had a tendency to produce greater amounts of audible noise, not having the “dampening” properties of a slowly controlled combustion in the IDI because of its pre-chamber initial combustion.) Many modern DI engines inject multiple times during one combustion cycle, starting a short combustion and building to the power-producing stroke helps quiet the common “clackity-clack of diesels. DI engines will also generate less heat and less volume of exhaust gas.

Two types of injectors

The oldest and most common type of diesel injector is the separate pump system. In it’s most basic explanation, this uses a separate injection pump, commonly indexed off of the timing chain, to turn what could be described as a mini-piston engine that “pumps” fuel to the various injectors that “pop” open when a specific or set pressure is reached. IDI injection pumps have a normal output fuel pressure of 2-4,000psi.

The latest style of injector is where there is no specific injection pump, but a prepump or helper-pump is used to bring the fuel at relatively low pressure to the injector itself and the injector has its own internal pump that increases the pressure to extremely high levels (up to 30,000psi and more!) These are used in DI engines.

Turbo chargers
Turbochargers are a type of forced induction system. They compress the air flowing into the engine. The advantage of compressing the air is that it lets the engine squeeze more air into a cylinder. More air means that more fuel can be added. Therefore, you get more power from each explosion in each cylinder. A turbocharged engine produces more power overall than the same engine without the charging, which can significantly improve the power-to-weight ratio for the engine.

A turbo consists of two “fans” on one shaft. One fan is driven by the exhaust gasses coming out of the engine (called the turbine), which drives the shaft and turns the compressor fan (the cold air intake fan) that compresses the intake air and pumps it into the intake manifold. The fan pulls in air from the air cleaner on one side and then it pushes it out into the intake manifold which supplies the cylinders with varying degrees of “boosted” air. A fan performs the function of moving air; however we are still left with the task of compressing the air. In order to compress the air; we must then contain it within an enclosed space (this is the compressor housing). Once the intake is compressed it gets sent to the engine via the intake manifold. This process of compression is what's technically referred to as "boost" (common boost pressures vary between 8-20 PSI with some big-rig diesels running 40 and more PSI!)

More "boost" means running more compressed air out of the turbo unit. This is usually related to the size of the unit itself. However, certain factors can limit the degree to which boost varies with the size of the unit. Turbos have a “waste-gate” that is calibrated to “dump” the exhaust pressure that the turbine fan sees so that only a certain RPM can be obtained (turbos typically run at 150,000 rpm and more), this gate is usually controlled via a feedback hose from the compressor housing, limiting boost to a certain pressure.

Why “Glow Plugs”?
Most diesel engines contain a “glow plug” of some sort. When a diesel engine is cold, the compression process may not raise the air to a high enough temperature to ignite the fuel. The glow plug is an electrically heated wire (think of the hot wires you see in a toaster) that helps ignite the fuel when the engine is cold so that the engine can start

Diesel fuel
Diesel fuel (including biodiesel and other fuel oils) evaporates more slowly because it is heavier. It contains more carbon atoms in longer chains than gasoline does (gasoline is typically C9H20, while diesel fuel is typically C14H30). It takes less refining to create diesel fuel, which is why it is generally cheaper than gasoline.

Diesel fuel has a higher energy density than gasoline. On average, 1 gallon (3.8 L) of diesel fuel contains approximately 155x106 joules (147,000 BTU), while 1 gallon of gasoline contains 132x106 joules (125,000 BTU). This, combined with the improved efficiency of diesel engines, explains why diesel engines get better mileage than equivalent gasoline engines.

Worlds most powerful diesel engine!

Total engine weight	2300 tons (crankshaft alone weighs 300 tons)
Length	89 feet
Height	44 feet
Maximum Power	108,920 hp at 102 rpm
Maximum torque	5,608,312 lb/ft

Fuel consumption at maximum power is 0.278 lbs per hp per hour (Brake Specific Fuel Consumption). Fuel consumption at maximum economy is 0.260 lbs/hp/hour. At maximum economy the engine exceeds 50% thermal efficiency. That is, more than 50% of the energy in the fuel in converted to motion.

For comparison, most automotive and small aircraft engines have BSFC figures in the 0.40-0.60 lbs/hp/hr range and 25-30% thermal efficiency range.

Even at it's most efficient power setting it consumes 1,660 gallons of heavy fuel oil per hour.