Featured Articles

What is Cackle ?
Part 1: Diesel Combustion Basics
by Kim Lux
Copyright© 2000, 2001 Diesel Research Inc. - all rights reserved

Author's Note:

The following is a simplistic discussion of some aspects of diesel combustion. While purists in the crowd will be able to point out several inconsistencies due to simplifications, it does provide a basic framework to start discussing more advanced concepts.

Introduction

The first step to understanding Powerstroke cackle is to understand how diesel engines operate and some of the details of their fuel injection systems. 

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. 

Putting Fuel Injection Physics Into Perspective

The combustion of diesel fuel in a diesel engine is a remarkable feat.

a) Fuel injectors must precisely meter a very small amount of fuel.

Most people probably don't realize how little fuel is injected into the combustion chamber of a Powerstroke when it is idling: 10 mm^3, which is a sphere about 2.67 mm across or 0.105 inches in diameter. This is nearly half the size of a 0.177 caliber BB. Furthermore, that amount must be the same from cylinder to cylinder. If one cylinder receives 15 mm^3 and the next one receives 5 mm^3, the engine will idle rough and probably knock.

Under full governor (not throttle although it is commonly used!) conditions, the same fuel injector might deliver over 150 mm^3 of fuel.

b) The fuel is injected at extremely high pressures.

The typical fuel pressure inside a Powerstroke injector is 17,500 PSI. (It is frequently advertised as 21,000 PSI, but that occurs only in special circumstances.) That pressure is the same as standing TWO fully loaded F250 SD trucks on the end of a 1.25" diameter rod. For comparison sake, a 40,000 PSI water jet is now used to cut steel in many special applications. 

c) The fuel is injected very quickly.

The entire power stroke of an engine running at 2400 RPM is only 12.5 milliseconds. In order to achieve efficient combustion, most of the fuel should be burnt in the first quarter of the stroke, resulting in an injection window of less than 3 milliseconds. How long is three milliseconds ?  It is roughly the time it takes your truck to travel 3 inches at 60 MPH.

d) The fuel injection is precisely timed.

For the engine to run efficiently, the injection must be timed to within a degree or so. At 2400 RPM, this equates to a margin of error of about 70 microseconds or less than a tenth of an inch at 60 MPH.

e) The amount of fuel supplied to each cylinder must be nearly the same.

Modern pickup diesel engines have 6 or 8 cylinders. For a smooth and efficient running engine, every cylinder in the engine must receive the same amount of fuel as any other cylinder, regardless of location.

f) The combustion chamber must be very turbulent

A typical stationary hydrocarbon/air mixture burns with a velocity of less than 1 meter per second. The distance across a typical combustion chamber is about 10 cm (4 inches). It would thus take over 100ms for the fuel in a combustion chamber to burn, if there was no mixing action of the air and fuel as it burnt. This would limit the speed of our engine to about 500 RPM, which would not suit our need for a compact, powerful engine. 

As the combustion stroke of an engine at 2400 RPM is about 12.5 mS, engine designers induce turbulent motion into the burning of the air fuel mixture so that combustion occurs much quicker than it would with a stationary mixture. Engine advertisers often speak of high swirl combustion chambers. What they mean is that the combustion air is given a high rate of twist as it enters the combustion chamber, allowing the air and fuel to burn quickly and efficiently. 

Trick Question Of The Day

Volumetric efficiency is the measure of an engine to totally fill its combustion chambers. For example, an engine with a volumetric efficiency of 80% is said to be filling its combustion chambers with 80% of the air it theoretically could. 

My friend Sam has a naturally aspirated diesel engine with a volumetric efficiency of 75%. Sam would like his engine to make more power. Sam's friend Richard just souped up his Big Block Ford 460 gasoline engine with high flow heads and headers. Richard did not "re-chip" his engines EFI computer. Richard claims that the volumetric efficiency went from 75% to 93%, resulting in about a 20% increase in power. Richard tells Sam that he should make similar modifications to the heads and exhaust system on his diesel engine so that it makes more power.

Question 1: What was the change in fuel delivered to Richards engine due to the modifications, assuming the air fuel mixture stayed the same?

Question 2: What would be the change in fuel delivered to Sam's engine due to performing similar modifications, i.e.: those that increase volumetric efficiency?

Question 3: If Sam did increase the volumetric efficiency of his diesel engine, what would happen to the air/fuel mixture?

Question 4: From the answers to questions 1 through 3, comment on the nature of engine modifications made to gasoline engines versus diesel engines to increase their power.

Question 5: What is the relative volumetric efficiency of Richard's engine at half throttle ?

Note: I've got a Powerstroke head sitting in my shop as I write this. In a future article I will provide some close up pictures and comment on the design of a Powerstroke head. I will answer, once and for all, if the fuel gallery is cast or drilled. I'll bet the fuel is boiling in the fuel rail... 

Kim Lux
Diesel Research, Inc.