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Old 06-25-2009
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Default Diesel Combustion Basics.

Here is in a nut shell.



Diesel Combustion Basics
The combustion process in a diesel engine is totally different from that of a gasoline engine.
On the intake stroke, a diesel engine draws pure un-throttled air into the combustion chamber. There is no fuel mixed with the air, as there is in a gasoline engine. (If you ever want to test a mechanic for simple diesel knowledge, ask him where the throttle plate is on your Powerstroke. He should tell you there is none.) On the compression stroke, the diesel piston compresses the air at a ratio of 17.5:1, resulting in a cylinder full of very hot (700F), highly pressurized (>500 PSI) air. (These numbers depend on a number of factors: inlet air temp/pressure, engine speed, engine temperature, etc.) With no fuel in the air and no ignition source, our engine has yet to make any power.
On the intake stroke, a gasoline engine draws a mixture of fuel (gasoline) and air into the combustion chamber. Once the fuel/air mixture is compressed, a sparkplug ignites the mixture, allowing the engine to make power. In a diesel engine, something must put fuel into the combustion chamber. We call that something a fuel injector.
Back to our diesel engine example: the piston is at the top of its stroke and the combustion chamber is full of hot, compressed air. The fuel injector injects a tiny bit of fuel into the combustion chamber as a very fine mist. The fuel mist is quickly heated by the hot compressed air and it begins to burn. Our engine makes power.
Diesel Combustion Technicalities
While the simple explanation provided above is all well and good, there are a number of "real world" details that enter into the picture:
a) Diesel engines are "throttled" by the amount of FUEL that is injected into the combustion chamber.
The speed and power of a gasoline engine is controlled by the amount of air/fuel mixture that is admitted into the combustion chamber. This is done with a throttle plate, which provides a partial vacuum above the cylinders, preventing them from filling to their fullest amount.
In a diesel engine, the engine speed and power is controlled by the amount of fuel injected into the combustion chamber. A diesel engine is not "throttled" as a gasoline engine is, but rather it is "governed" by a device that controls how much fuel the fuel injector injects. Injecting a greater amount of fuel into the combustion chamber will allow the engine to develop more power.
b) Because diesel engines are governed, the mixture does not burn at stoichiometric ratios.
In a gasoline engine, the air/fuel mixture always burns at 14.7 pounds of air to one pound of fuel, or nearly so. When the engine is throttled, less of the air fuel mixture is admitted to the combustion chamber, but the ratio of the air to fuel in the mixture is always nearly 14.7:1. In fact, it is impossible by conventional means to burn a air/fuel mixture much different from 14.7:1 in a spark ignition engine.
In a diesel engine, the combustion chamber is always full of pure air before the fuel is injected. When the engine is idling under load, a very tiny amount of fuel is injected. When working hard, say while pulling a big load, a lot of fuel (relatively) is injected. Because the amount (weight) of air in the combustion chamber is relatively constant (I'm not considering boost here...) and the amount of fuel is variable, diesel engines run at varying air/fuel ratios. At idle, with no load, it is not uncommon to have a diesel engine running at an air/fuel ratio of 60 or 100:1. Under full power, most diesel engines need to run lean of stoichiometric. (BTW: the stoichiometric ratio for diesel fuel is NOT 14.7:1 and it varies slightly depending upon the composition of the fuel.)
For a number of reasons, most diesel engines will emit visible hydrocarbons (i.e.: smoke) if run near or over their stoichiometric fuel ratio. Diesel engines thus always need to be run lean of stoichiometric.
On some gasoline engines, certain cylinders have a habit of running leaner than others, due to something called fuel dropout in the intake manifold. A condition can therefore develop where one cylinder is too lean to fire properly at all, resulting in something that people term a "lean misfire". Because diesel engines are designed to run lean of stoichiometric, such an event can NOT happen in a diesel engine.
c) The injected fuel isn't injected all at once
While a fuel injector injects fuel quite quickly (usually taking less than a few milliseconds), it does not inject the fuel instantaneously. Big deal ? What is the difference between taking a few milliseconds and injecting the fuel instantaneously ? Lots ! Because...
d) The injected fuel doesn't burn right away
When the fuel is injected into the combustion chamber, it doesn't begin burning immediately. First, it mixes with the air swirling around in the combustion chamber. Then, it heats up a bit until it reaches its ignition point. Then it starts to burn at the periphery of larger fuel concentrations. All these processes take time. Not a lot of time, but some time none the less. Once ignition begins, the pressure, temperature and turbulence in the combustion chamber increase and the burning process speeds up dramatically. Soon, the larger fuel concentrations will start burning. Soon the combustion chamber is hot and turbulent enough that the fuel does ignite almost instantaneously. The time it takes for fuel to start burning after it is first injected is called ignition lag.
Ignition lag is a hugely important issue in diesel engine design. Most fuel injection systems deliver fuel at a fixed rate: they don't have the ability to start injecting fuel slowly and then speed up, for example.
Lets use an example to see why ignition lag is important. Lets assume that the ignition lag of a certain combustion chamber under a certain load is 100 micro seconds. (That is 100 millionths of a second.) Lets also assume that the entire injection duration is going to be 1 millisecond. (That is 1 one thousandth of a second or 0.001 seconds.) If our injection system has a fixed fuel injection rate, 10% of the entire fuel charge will have accumulated in the combustion chamber before ignition occurs. (100 microseconds/ 1millisecond = 10%). If the ignition lag time is 200 microseconds, 20% of the entire fuel charge will have accumulated in the combustion chamber.
Having a large amount of accumulated fuel in the combustion chamber when combustion commences is a bad thing, for a number of reasons:
I) It increases emissions, particularly NOx. Generally speaking, NOx is created when an oxygen/nitrogen mixture is subjected to high temperatures and pressures. At the start of combustion, the combustion chamber on a diesel engine is filled with air, which is a mixture of oxygen and nitrogen. It is under high pressure. It is fairly hot. If a large amount of accumulated fuel suddenly ignites, creating a very hot flame front, the process will probably create a large amount of NOx as well.
II) It makes the engine noisy. When the fuel does combust, the more there is of it, the bigger the bang. Technically speaking, a large amount of accumulated fuel quickly combusting will result in a rapid increase in cylinder pressure, something which humans perceive as the characteristic diesel knock sound.
III) It provides localized heating of the combustion chamber. If a small amount of fuel is accumulated when combustion starts, it will more easily mix with the air in the combustion chamber to minimize combustion chamber hot spots. When a large amount of accumulated fuel is present and burns quickly, there is less time to mix with the air and localized heating is more prevalent.
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