Homogenous Charge Compression Ignition (HCCI)
It’s hard to believe that the internal combustion engine has been around for over 200 years. The design of such an engine is a combination of work from many different individuals, but basically we attribute the modern engine to Nikolas Otto. Nickolas was a German engineer who developed the compression charge internal combustion engine that ran on liquid petroleum gas. This basic design is what the modern engine is based from. Over the years many individuals have put their twist on Nickolas’s engine design in order to enhance the reliability and performance.
As you already know the modern internal combustion engine has seen a few twists. These twists are based on technological advancements in order to produce better performance and emission production. But perhaps you are not aware of one advancement the modern engine has seen, the Homogenous Charge Compression Ignition (HCCI) engine. Homogenous Charge (HC) refers to the charge state prior to ignition. A substance is homogeneous if its composition is identical wherever you sample it. This means that the charge mixture (fuel and air) has a uniform composition throughout the cylinder. Compression ignition (CI) refers to the method that is used to drive the fuel past its autoignition point. 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 vibrational energy of the atoms causing a heating effect. This process is called Adiabatic Compression. The Adiabatic processes are characterized by zero heat transfer with the surroundings. In the case of rapid compression, the process occurs too fast for any heat transfer to occur. Heat transfer is a slow process. This rapid compression of the air creates a rapid heat increase that is used to drive the fuel well past its autoignition point.
There are multiple ways used to combust fuel in the internal combustion engine. The fuel stock that is selected will set the method that will be used. In the automotive industry the fuels that we are most familiar with are Gasoline and Diesel. These fuel stocks have been around for many years and are used around the world. When using these fuel stocks the ignition point is obtained with different methods. Gasoline will use the method of spark ignition, while Diesel will use the method of compression ignition.
In the spark ignition method the charge prior to ignition is that of a homogenous charge. This means that the fuel air 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 79% nitrogen, which is used as the working fluid, and 21% oxygen which is used as the oxidant. The reaction will occur between the fuel, which is hydrocarbon based, and 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 neither chemical will 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 there will not be a complete reaction between all of the chemicals 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.
In a spark ignition system the spark provides the heat that will push the temperature above the autoignition temperature of the fuel. The fuel air mixture is homogeneous and is kept very close to stoichiometry. As previously discussed this is the exact amount of fuel and air necessary for complete combustion of the fuel. The spark provides the point of ignition within the cylinder. This is the point where the fuel stock is driven past the autoignition temperature in a localized area around the spark event. This point can be controlled as the start of combustion, however the combustion phenomena itself is much harder to control. The combustion phase of the charge is where the chemical energy is changed 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 and is driven by the transfer of heat. When the flame front is propagating across the combustion chamber the fuel and air, being a homogenous charge, allows this flame front to move without being hindered. This allows for a stable combustion event of the fuel. Any attempt to improve fuel economy by running a lean mixture with a homogeneous charge will result in unstable combustion. This is due to the propagation of the flame front being impeded. This will impact the power production and emissions of the engine.
In the compression ignition method the charge prior to ignition is that of a stratified charge. A stratified charge refers to the state of charge as having gradients or layers. This is where the fuel air charge is not mixed, but is separated. The cylinder is charged with air and the fuel is injected directly into the cylinder as a fuel rich aerosol concentration as seen in Figure 2. This is usually a mixture with a lean stoichiometric ratio. The air volume within the cylinder having been compressed is heated well above the autoignition point of the fuel stock. When the fuel is injected into the hot air within the cylinder the fuel changes states from a liquid to a vapor. The heat continues to be driven into the fuel within the cylinder thus establishing the point of ignition. This is the point where the fuel stock is driven past the autoignition temperature in a localized area around the injection event. This injection point can be controlled as the start of combustion, however the combustion phenomena itself is much harder to control. The injected fuel plume has gradients of fuel mixture, as well as gradients of temperature and pressure. The fuel plume, being a rich vapor mixture, will only burn at the surface. In order for the chemical reaction to occur the air concentration surrounding the fuel plume must interact with the fuel at a surface level. The surface level of the fuel plume is at a stoichiometric ratio, which allows the chemical reaction to take place.
During the combustion event the air on the surface of the fuel plume, and the fuel on the surface of the fuel plume are combusted or burned. This combustion process is accomplished in layers. As the outer layer is burned the flame front is then driven into the next layer of the fuel plume. Burning in layers allows the fuel to burn more slowly, releasing its energy over more degrees of crankshaft rotation and thus producing more torque from the crankshaft. This combustion process continues until all of the fuel has been consumed within the reaction. If the fuel plume is too rich then the fuel in the center of the plume will not have enough oxygen to burn. This will create black smoke with heavy particular matter expelled from the exhaust system.
A stratified charge can allow higher compression ratios to be run without creating pre-ignition. There cannot be pre-ignition without fuel in the cylinder. Stratification allows the fuel to be injected only when the piston is close to the Top Dead Center (TDC) position. This late injection timing will set up the fuel air charge for ignition. Additionally the way in which the fuel is burned on the boundary layer of the fuel plume, the engine can run at a much leaner air/fuel ratio than that of a homogenous charge engine.
HCCI is not a new development but has been around for many years. What is new is the way that the HCCI system is implemented and the electronic control of the HCCI system. HCCI uses the Otto combustion cycle (4 stroke engine) and was popular before the introduction of spark ignition. Since the internal combustion engine is a heat engine, the fundamental operation of the device is the production and use of heat. The Otto cycle describes the idealized thermodynamic operation of the 4 stroke engine, shown in Figure 3. In these engines everything that is done prior to the combustion of the fuel type is to set up the fuel air in the cylinder so the charge can be ignited, burned, and combusted. The conversion of chemical potential to thermal energy is important. The way in which this conversion takes place can change the engine’s thermodynamic efficiency. The engine’s thermodynamic efficiency is a measure of how effectively the engine converts heat into mechanical power. As we have seen, the way in which the fuel air charge is set up and ignited is quite different and will change the thermodynamic efficiency of the engine. What is needed is a way to best extract the energy from the fuel stock. This is where HCCI comes into play. The HCCI engine provides a different way in which the fuel air charge is set up and combusted.
The basis for the HCCI operation is the Homogenous Charge (HC). This is where the fuel air charge is that of a uniformed composition throughout the cylinder. In a gasoline based engine the most popular method is to deliver the fuel with a port style fuel injector and mix the fuel and air prior to the entrance into the combustion chamber. Yet another way to fuel the engine is to use direct injection (DI). With DI the fuel is injected with fine liquid fuel droplets directly into the combustion chamber. This method has less time to mix the fuel and air so it is much harder to obtain a mixture that is completely homogenous. However recent advancements have been made that allows the DI to obtain a better mixture within the cylinder. The port style fuel injector delivers a gaseous suspension of fine liquid fuel droplets that can be suspended in the air flow moving through the engine’s induction system and into the cylinder. As this mixture moves into the cylinder the mixing can be increased by a tumble and swirl effect which creates a fuel air mixture that is nearly homogeneous throughout the cylinder prior to ignition. This aerosol mixture is in a liquid format that once injected and moved into the cylinder is heated with adiabatic compression. This heat will change the liquid gasoline into a gasoline vapor that can be combusted. If the compression of the air is high enough the heat will continue to be driven into the fuel, this in turn will drive the gasoline past its autoignition temperature and will start the combustion process.
The HCCI combustion process is different from a spark ignition gasoline engine. In a spark ignition gasoline engine the point source of the ignition event causes a non-uniformity within the combustion chamber as a function of the fuel burn process. During the combustion process in an HCCI engine the fuel throughout the cylinder is heated and ignited near simultaneously. This is not an explosion, but instead a somewhat controlled energy release of the fuel. This near simultaneously ignition event provides for a more rapid heat release from the combustion event that increases the peak pressure within the cylinder as seen in Figure 4. This in turn increases the engine’s thermodynamic efficiency.
The HCCI Combustion event has benefits over Homogenous Charge Spark Ignition HCSI) and Diesel Direct Injection Ignition (DDII). With spark ignition combustion in order to achieve a complete burn the fuel air mixture needs to be close to that of stoichiometric. With HCCI combustion the mixture can be lean of stoichiometric and still achieve a complete burn with low nitrogen oxide (NOx) production. Diesel can also achieve a complete burn lean of stoichiometric, however, due to the way the fuel plume is burned particular matter (soot) is formed with high NOx emissions. Only the HCCI combustion process provides multiple ignition points throughout the cylinder as seen in Figure 5. Unlike conventional combustion in the spark ignition or diesel ignition process, HCCI does not rely on a flame front to propagate combustion but instead combustion occurs as spontaneous ignition everywhere in the charge volume when the required conditions are met.
Gasoline’s autoignition temperature is much higher than that of diesel fuel. Gasoline has a flash point of -45°F, and an autoignition point of 536°F. Whereas diesel fuel has a flash point of 126°F, and an autoignition point of 256°F. In order to autoignite gasoline the compression will need to be higher than what is needed for diesel fuel. To autoignite diesel fuel you need a compression ratio of 11.0:1. In order to start a cold diesel engine the compression ratio will need to be much higher. To insure a cold diesel engine can start, the compression ratio on the direct injection diesel engine is usually 18:1 to 24:1. In order to autoignite gasoline the compression ratio will need to be 15:1 or higher. At part load the compression ratio to autoignite the gasoline will need to be at least 17:1.
With a need for a higher compression ratio to enable the HCCI combustion event, the engine will need a way in which this can be obtained. One way is to use an engine with a Variable compression Ratio (see April 2018 VCR A Future Technology Applied Today). This is where the compression can be controlled through changing the static compression ratio of the engine. Another way to control the compression charge rate would be the use of forced air induction. One method would be the use a supercharger with a constant variable control pulley drive system. With this system the supercharger can spin at speeds that are different from that of the crankshaft speed. This allows the cylinder charge volume to be controlled, an increase in volume raises the compression ratio whereas a decrease in the volume lowers the compression ratio.
Yet another way to heat the cylinder volume is through exhaust gas dilution. This can be done by rebreathing or by recompression. In rebreathing, the cylinder charge temperature is controlled by exhaust gases being cycled back into the combustion chamber after exiting the exhaust port. In recompression the cylinder charge temperature is controlled by trapping hot residual gas from the previous engine cycle by closing the exhaust valve early during the exhaust stroke. In either method the charge temperature can be controlled.
Now that the charge temperature can be controlled for the autoignition of the fuel, the fuel itself can be a problem for this process. Gasoline is a mixture of hydrocarbons (HC) components. These HC base chemicals have different hydrogen carbon bonds with different chain configurations and lengths. This allows some of the HC chemicals to vaporize at low temperature, while other HC chemicals vaporize at higher temperatures. Additionally the flash points and autoignition points of these HC chemicals will vary widely with temperature. One would think that there would be a standard recipe used when blending gasoline, however this is not true. Basically any hydrocarbon that burns can be utilized in the gasoline blend. There are laws that the gasoline will be required to meet for the pump rating of the fuel and the volatility of the fuel. The pump number is how the fuel is rated to control engine knock and the Reid vapor pressure is a rating of how much of the liquid fuel vaporizes at a given temperature. Both of these ratings are important for the performance and emissions of the engine.
Since there is no prescribed way to blend the fuel base, each gas station will have different hydrocarbons in their fuel blend. This becomes a problem with the HCCI combustion event. Since there is not a given start of the combustion event, the heat within the cylinder is what will dictate the point of ignition. Therefore the heat within the cylinder will have to be managed very carefully. Different hydrocarbons will vaporize and ignite at different temperatures, thus each gas station will have fuel that will have different autoignition points. If the autoignition points of the fuel are too low detonation can occur. Detonation within these engines will have to be avoided at all costs. Detonation is a supersonic shockwave that occurs throughout the combustion chamber creating a near stepwise change in pressure. This is where the charge is ignited instantly. Detonation can cause catastrophic damage to the engine.
These fuel differences will make the operation of the HCCI engine very difficult. In order to have better control over the entire operating range of the engine a dual mode can be used. This will allow the spark ignition on gasoline based engines, or diesel direct injection ignition on diesel based engines, to remain operational under some conditions. For example a gasoline based engine will idle with spark ignition combustion then under light to moderate load the engine will operate with HCCI combustion and at heavy load the engine will operate with spark ignition.
However there are problems when using dual mode ignition. For instance when transitioning to and from homogenous charge spark ignition (HCSI) and homogenous charge compression ignition (HCCI) modes of engine operation. HCCI under part load requires a compression ratio of 17:1. When transitioning from HCCI to HCSI the compression ratio will need to change almost instantaneously to 12:1. This will be difficult to accomplish with just a variable compression ratio engine. So the variable compression ratio will only be used for part of the heating of the air charge. Boost pressure form force air induction and exhaust dilution will be used in conjunction with the variable compression ratio. The results are far better when using multiple systems to control the temperature of the air charge.
The use of HCCI has many practical benefits. These engines operate close to the maximum thermal conditions for the engine. Higher compression ratios provide greater performance with better emission and fuel economy. HCCI improves the conversion efficiency which improves the thermodynamic efficiency of the engine. HCCI provides for faster laminar flame front temperatures, thus higher combustion chamber peak pressures without the production of NOx. Faster combustion rates increase engine response. Additionally running less ignition advance lowers parasitic losses. HCCI can also run lean of stoichiometric while still providing a complete burn of the fuel. As you can see these benefits of the HCCI combustion process are significant. This means that these engine will soon be in production and in your service bays.
Always enjoy your in-depth discussions of functionality and purpose. Thanks Bernie!
Thanks Bernie! I've learned a lot from you over the years. Can you explain the the difference between flash point and autoignition temperature?
Fantastic information. Thank you for taking the time to contribute this!
The flash point is the temperature that a substance, that is in a vapor format, will ignite when exposed to an ignition source (e.g. open flame). The autoignition temperature is the temperature where a substance, that is in a vapor format, will spontaneously ignite without an ignition source.
Thank Bernie! Put that way it's almost self explanatory!
Very well written, as is the norm with you Bernie. Excellent information in every detail.