Welcome to Part 4 of the Teaching Engine Thermodynamics Series. In this edition, it’s all about FEELING THE BURN!

Combustion

In a gasoline engine, a spark plug is used to ignite a mixture of fuel (gasoline) and air.  This ignition takes place during the “combustion” step shown in the Otto Diagram. The ignition’s purpose is to create temperature and pressure in the cylinder, which later drives the piston downwards during the Power Stroke. During the combustion step, the cylinder is at Top Dead Center (approximately), and both the intake and exhaust valves are closed.

Burning gasoline to create temperature and pressure seems like a straight forward process, however it is actually a complicated balance of variables. The timing of the spark, amount of air, amount of fuel, and the type of fuel are all important variables to keep an engine running smoothly and efficiently.

Air to Fuel Ratio

Understanding Why and How combustion takes place is absolutely critical to understanding and optimizing your car’s engine. The most fundamental aspect of combustion is the ratio of oxygen and fuel since combustion is actually a rapid chemical reaction of oxygen and fuel. The lowest concentration of a fuel in air capable of burning in presence of an ignition source is called the Lower Flammability Limit (LFL). Having less fuel than the LFL will not support combustion. Similarly, the highest concentration of fuel in air capable of burning is called the Upper Flammability limit (UFL). A mixture with more fuel than the UFL is called “Rich” and a mixture with less fuel than the LFL is called “Lean.” Rich and Lean are terms common to the automobile, and oxygen sensors and other sensors are present on any modern car to control the ratio of air and fuel within desirable limits. Sometimes the LFL is called the Lower Explosive Limit (LEL), and theUFL is called the Upper Explosive Limit (UEL).

For gasoline specifically, there is a stoichiometric amount of fuel that will be completely consumed with one part of air. This is called the stoichiometric ratio, and happens to be 14.7 parts of air to one part of fuel. To get the most amount of energy from fuel, you would want to combust every molecule of it, which will only occur at the stoichiometric ratio or leaner. This is why oxygen sensors are so important on a car. Without them, the computer would not know if you were lean or rich, and would have to guess or use defaults that may not always be accurate.

All this talk of air to fuel ratio brings up a good point to make. If 14.7 parts of air are required for each part of fuel, then it’s clear that a-lot more air is needed than fuel. This is the reason that air intake is almost always the bottle neck on a car engine. Installing larger fuel pumps, injectors etc won’t do much good if you can’t get more air to the engine. At best, there would be no impact. At worst, the vehicle would needlessly run rich and increase fuel consumption without adding any extra power.

Ignition Timing

In a gasoline engine, the fuel requires a heat source to ignite. In this case, a spark plug is used. High voltage is used to create an electric arc in the electrode of a spark plug. This electric arc creates enough heat to ignite the blend of fuel and air. When a car engine is running, the internals of the engine are always moving. Since the pistons and valves are always moving, the timing that this spark occurs is important. If the fuel was consumed instantly when the spark occurred, then Ideally we want the spark to occur exactly when the piston is at top dead center after the compression stroke. At this time, the cylinder as at peak pressure, and since it is about to start moving downwards, the energy from the combustion would all contribute to the power stroke.

However, in the real world conditions are not so ideal. Actually, the combustion process takes some time to occur, typically around 30 milliseconds. If we started the combustion process at top dead center, most of the fuel would actually combust about 15 milliseconds after the cylinder reaches top dead center. This may not sounds like a-lot, however in an engine turning at 3,000 RPM, this combustion would happen almost 25% of the way through the power stroke. For this reason, engines use something called “Ignition Advance”, which is a critical component of a properly tuned engine.

Ignition advance takes into account the time required to burn gasoline, and fires the spark plug that much earlier. However, problems will also occur if ignition occurs too early. Before the combustion process, the piston is completing it’s move up the cylinder during the compression stroke. To do this, the piston is compressing air and fuel in the cylinder, which requires a great deal work work to complete. If the combustion process occurs too early, it will build cylinder pressure during the compression stroke and fight against the piston while it’s moving upwards, rather than helping the piston move downwards during the power stroke. This would severely reduce engine power and efficiency, and can actually damage pistons. early ignition could be caused by too much spark advance, however it can also be caused by fuel combusting on its own during the compression stroke. This is called pre-ignition, spark knock, or pinging. The goal is to balance the timing of the burn, so that the majority of the energy from combustion occurs as early in the power stroke as possible, while limiting the amount of pressure generated by ignition during the compression stroke.

So, the combustion process takes a fixed amount of time, lets say 30 milliseconds. This means, if the engine is rotating at 1,000 rpm, there are a certain number of degrees of engine rotation required to account for those 30 milliseconds. What happens if the engine starts rotating at 3,000 rpm? More degrees of rotation (advance) will be required to accommodate the same 30 milliseconds. This is the basis of a technique called the “Advance Curve”. Basically, more advance is programmed into the computer at higher engine speeds. This ensures that more ignition timing will always be optimum. What about throttle position? We mentioned earlier that the fuel to air ratio changes depending on throttle position. The rate of combustion changes depending on the amount of fuel present. At full throttle, the combustion process may take an extra few milliseconds because more fuel is present, and therefore more ignition advance is required. To obtain optimum energy efficiency, an engine needs to be account for these differences. On older engines, vacuum advance was used to account for throttle position (and therefore air to fuel ratio differences). On newer engines, this is all done electronically in the computer. These two tweaks to ignition timing combined are called the “ignition advance curve.”

All of the above non-ideal aspects of combustion result in a change to the ideal Otto cycle. Since some of the combustion process occurs during the compression stroke due to ignition advance, and some of the combustion process occurs throughout the early parts of the power stroke, the curve in the Otto cycle becomes a more rounded shape, rather than the ideal vertical line. These are shown below.

Fuel Blend

Is all gasoline created equally? NO! But this is not a debate about gas Brands. Many people don’t realize that the gas in your city likely came from the exact same place, regardless of the name on the gas station. However, fuel does change seasonally and octane rating is important.

Why is octane rating important?

Octane rating is determined by the formula (R+M)/2. The octane rating number is defined as a value used to indicate the resistance of gasoline to spark knock. Octane numbers are based on a scale on which isooctane is 100 (minimal knock) and heptane is 0 (bad knock). A gasoline with an octane number of 92 has the same knock as a mixture of 92% isooctane and 8% heptane.

So why does spark knock occur? Gasoline has a temperature at which it will combust on it’s own in the presence of oxygen. This is called the “Auto ignition Temperature”. When a gas is compressed adiabatically, the temperature rises due to the second law of thermodynamics. During the compression stroke, the gas inside the cylinder is a mixture of fuel and oxygen, and is compressed approximately 9x. This generates a-lot of heat. If a fuel has an auto ignition temperature low enough, it will combust all on its own during the compression step, and cause spark knock. Rather, we want the fuel to combust when we fire the spark plug, so that we can take advantage of our precise ignition timing. To combat this, fuels with a higher auto ignition temperature can be used.

Is higher octane fuel better? Not necessarily! You should only use a fuel with an octane rating that is the minimum of what you need. For example, if you require 87 octane fuel in your engine, running 89 octane fuel will not help you in any way. In fact, high octane fuel often contains more ethanol to increase the octane rating, which actually reduces your miles per gallon. High octane fuels also contain other blend additives, such as xylenes to increase the octane rating. These additives can require longer to burn, which requires additional ignition advance, which moves more of the energy from the combustion process into the compression stroke, and reduces engine efficiency slightly. Not to mention high octane fuel costs more money. Bottom line, if you buy fuel with a higher octane rating than required, you are wasting your money.

Engine Design

There are a number of engine advancements that have improved the combustion process. One such advancement is the location of the spark plug. By precisely placing the spark plug’s electrode in the cylinder, you can improve the rate of combustion and the temperature of combustion. Such an advancement was made on the Chevy Vortec cylinder heads verses the traditional Gen 1 SBC 350 heads.

“Direct Injection” is also a new technology that is becoming more common. The fuel is injected directly into the combustion chamber rather than into the intake manifold. By doing this, the timing and the location of the fuel can be controlled more closely, which allows for a more efficient combustion process.

We hope you enjoyed this edition of the Car Engine Thermodynamics series! Check out the next part for more brainy car knowledge!

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