The internal combustion engine has been around for over 200 years. In this time, there have been many changes to the engine, the fuel, and the automobile. We attribute the modern engine to Nikolaus Otto. Nikolaus was a German engineer who developed the compression charge internal combustion engine that ran on liquid petroleum gas. Nikolaus’ engineering marvel is still used to power the modern vehicle.
|Want more ? Enjoy a free subscription to Motor Age magazine to get the latest news in service repair. Click here to start you subscription today.|
First, some theory
The fuel stock and the internal combustion engine have undergone some changes in the past years, but the basics are still the same. The fuel stock that we will cover is a liquid petroleum product that we refer to as gasoline. Modern gasoline is a mixture of different chemical components with varying vapor points and varying auto-ignition temperatures. Basically when these components are mixed together and form gasoline, they have an approximate flash point of -45°F and an approximate auto-ignition point of 536°F.
It will be necessary to understand that liquid gasoline cannot burn in this state (liquids do not burn). In order to burn gasoline it must be heated so that it makes a phase transition and turns into a vapor (vapors can be burned). The compression within the cylinder accomplishes the heating of the fuel. When air is compressed rapidly the molecules are accelerated off of the moving piston where they hit one another. The kinetic energy from the piston is turned into thermal energy in the air charge. This occurs from the atoms hitting one another which in turn starts the atoms vibrating, causing a heating effect. This process is called Adiabatic Compression. The Adiabatic processes are characterized by zero heat transfer with the surroundings, such as the piston, cylinder and cylinder head. In the case of rapid compression, the process occurs too quickly for any heat transfer to occur to these components. Heat transfer is a slow process. This rapid compression of the air creates a rapid heat increase within the air charge. Thus this heat increase is put into the fuel that is suspended within the air. When this air/fuel charge is heated it turns the fuel into a vapor that can be burned.
Now that the fuel is in a vapor format and is ready to burn, a spark takes place across the sparkplug electrode. The spark ionizes the spark plug electrodes producing a state of plasma which takes the fuel well past the auto-ignition temperature of the fuel; setting up the ignition phase of the fuel. This is where the temperature in a localized area around the sparkplug starts to burn. The next stage is the combustion phase. This is where the charge changes from chemical energy to thermal energy. The heat released is then driven into the next layer of the charge thus igniting it. This is referred to as deflagration. Deflagration is the combustion that propagates at subsonic speeds through the gas that is driven by the transfer of heat. This heat transfer heats the working fluid (nitrogen) which in turn puts pressure on the piston, thus pushing the piston down the cylinder.
In the spark ignition method the charge prior to ignition is that of a homogenous charge. This means that the air/fuel charge is evenly mixed throughout the cylinder volume. In order to completely burn an evenly distributed mixture within the cylinder, the air/fuel ratio must be very close to that of stoichiometry. Stoichiometry refers to the weights of the chemicals that will react. In an internal combustion engine the fuel is the reactant and the air is the oxidant. Air is comprised of approximately 78.09 percent nitrogen, which is used as the working fluid, and 20.95 percent oxygen which is used as the oxidant. The reaction will occur between the fuel, which is hydrocarbon based, and the oxidant, which is the oxygen. The stoichiometric ratio between the fuel and air is one where the hydrocarbons and oxygen are at a weight ratio that, once they react with one another, will no longer be present. This means that the hydrocarbons break apart becoming hydrogen and carbon. In the presence of oxygen, the hydrogen combines with the oxygen forming a new chemical; dihydrogen monoxide (H2O water). The carbon attaches to the oxygen forming a new chemical; carbon dioxide (CO2). If the hydrocarbons and oxygen are at a stoichiometric ratio and react with one another then neither of these chemicals will remain present within the combustion gases, see Figure 1. The chemical weight will be the same but the new chemicals formed during a complete reaction will be water and carbon dioxide. Although the mixture is at a stoichiometric ratio, in the real world a complete reaction between all of the chemicals does not occur so there will always be some hydrocarbons and oxygen left after the combustion process. This is due to the flame front being unable to get into the crevasses around the spark plug, valve pockets, and piston rings.
If the cylinder compression is present, the fuel was vaporized, the air/fuel ratio was that of stoichiometric, the cylinder was homogeneous and the spark occurred correctly, the vast majority of fuel and air will react with one another. When this occurs the tailpipe gas charge will have high CO2 (> 14 percent), low O2 (< 1 percent), low CO (< 1 percent), and low HC (< 100 Parts Per Million (PPM)), as seen in Figure 2.
Analyzing that comes out
Figure 2 also shows an engine with no problems on start and run. Since the hydrocarbons react with the oxygen then the hydrocarbon level will drop, the oxygen level will start at atmospheric condition at about 21 percent and drop sharply, the carbon dioxide will rise sharply and the carbon monoxide will drop as well. At this point the catalyst (catalytic converter) is not hot enough to further the reaction of the fuel. There will be more on this later.