The primary difference between a diesel and HCCI engine is the nature of the air/fuel mixture. While a diesel typically uses what's called a stratified charge, the mixture in the HCCI is homogeneous. With a stratified charge, the fuel is concentrated in one area of the cylinder and the flame expands as the fuel burns. In order to prevent the mixture from igniting too soon diesel fuel is used since it has better resistance to self-ignition than gasoline. An HCCI engine can operate on a range of fuels including gasoline and ethanol and provide much of the fuel efficiency benefit of a diesel at lower cost and emissions. The downside of HCCI is that it's hard to use outside of a limited operating range. GM powertrain engineers have made significant strides in this respect and we had a chance to sample their work at the GM Tech Center this week.
When the media first drove GM's HCCI prototypes at the Milford Proving Ground in summer 2007, the operating range was limited to light loads between about 20 and 50 mph. Outside of that range, the engines operate as normal spark ignition engines. In order for HCCI to work the pressure and temperature in the combustion chamber has to be carefully controlled. If the temperature is too high, the air fuel mixture will ignite too early causing the clattering noise normally associated with diesel engines as well as potential engine damage. If the temperature is too low, the fuel will not burn properly. The original operating range fell into the HCCI sweet spot.
Before we reveal how GM managed to expand the operating range, let's review the basic operation of the GM HCCI system. The current prototypes are based on GM's 2.2-liter EcoTec four cylinder engine. A two step variable valve lift system and electric cam phasing on both cam-shafts is used to provide the different cam profiles needed for HCCI and spark-ignition operation. The same direct injection system used on the four cylinder engine in the 2010 Chevy Equinox provides fuel delivery and a pressure sensor is added to each combustion chamber for closed loop control.
The first change to the valve timing for the HCCI mode is to go to a lower lift configuration and close the valve much earlier than it does in SI mode, about 60 degrees before the piston reaches top dead center (TDC). The result is that some of the exhaust gases are retained in the combustion chamber which helps keep the temperature up. After the exhaust valve closes, the injector provides an initial burst of fuel that mixes with the exhaust gases. As the exhaust gases are compressed by the rising piston, the pressure builds and then falls as it helps push the piston back down.
In HCCI mode, the intake valve is also opened later than usual, about 60 degrees past TDC with lower lift. The lack of overlap between intake and exhaust reduces the occurrence of unburned fuel heading out the exhaust. As the intake valve is opening, a second larger burst of fuel is sprayed into the cylinder and the incoming air helps to mix it thoroughly resulting in a homogeneous mixture and fuel and air. As the piston is approaching TDC on the compression stroke, the pressure increase raises the mixture temperature to a point where it spontaneously ignites and virtually the entire mixture burns simultaneously.
With spark ignition, the flame starts at a single point around the spark plug and the flame front propagates around the cylinder. This type of combustion results in the fuel burning at about 2,200-2,300 K. HCCI combustion occurs at about 1,800-1,900 K putting it below the threshold at which most NOx is formed. Thus emissions of NOx during HCCI operation are reduced by more than 90% compared to the emissions during SI operation. That means an HCCI engine virtually eliminates the bane of diesel engines, NOx emissions without using expensive after-treatment systems like urea injection.
Because compression ignition relies on generating enough heat in the cylinder to trigger spontaneous ignition of the fuel, HCCI engines have had difficulty running at low speeds in the past because the exhaust gas retained by the early valve closure looses too much heat. GM's engineers have developed a mixed mode operation scheme for low speeds. A third small injection pulse is used just before TDC creating a localized fuel rich region. The spark is fired to ignite this rich zone which generates enough additional heat to cause the homogeneous combustion of the rest of the fuel.
As speeds increase, the temperatures in the combustion chamber increase, causing the homogeneous mixture to ignite prematurely. In order to better regulate the cylinder temperatures, the engineers have added an external EGR system to put some cooled exhaust gas back into the cylinder, bringing the temperature of the mixture back down to the optimal level for HCCI combustion.
The improvements in efficiency for HCCI come from several phenomena. The modified valve timing with no overlap results in lower pumping losses. The reduced combustion temperature is another factor. Because the fuel burns at a lower temperature, the temperature gradient between the combustion chamber and coolant is reduced. This results in reduced heat transfer, retaining more of it in the combustion chamber where it can do work pushing the piston down. The leaner homogeneous mixture also results in more complete combustion.
The key to managing all of this and getting HCCI to work consistently is a sophisticated closed loop control system. A sensor reads the pressure in the combustion chamber during each power stroke based on the crankshaft position. For optimal HCCI combustion, the pressure will be within a fairly narrow range and the powertrain control can adjust elements like injection pulses and valve timing. The air-fuel ratio during HCCI are typically much leaner than during normal SI operation often upwards of 20:1. In SI engines such lean combustion, is good for efficiency but produces more NOx due to the higher temperatures. The technologies that are now becoming mainstream like direct injection and variable valve timing and lift are making all of this possible today.
The first cell contains a single cylinder optical engine that allows the engineers to closely observe the combustion process itself. Single cylinder engines are often used to test different design ideas because fewer parts need to be machined and changes can be made more quickly. Once the basics are narrowed down the design elements can be built up into a full engine. The engineers can quickly swap out different cylinder bores of various lengths and diameters as well as different cylinder heads and crankshafts. The bores and pistons actually have windows cut into them that are filled with quartz allowing the combustion inside to be observed with high speed cameras. Quartz is used because it can withstand the pressures of combustion and the heat transfer properties are close enough to the metal parts to minimize the impact on the process.
The cylinder head on the test engine had a cut down version of the design used on the full four cylinder engine but instead of camshafts it uses electro-hydraulic actuators for the valves. This allows the engineers to quickly reprogram valve timing and lift profiles with infinite variability without having to change components. Once an acceptable profile is determined, conventional camshafts are ground for testing. Next to the optical engine cell is a second cell that features single and four cylinder engines also running with electro-hydraulic valve actuation for additional calibration. One of the primary areas of interest right now is the transition between HCCI and SI modes to make it as seamless as possible. Finally a third cell is used for testing engines with full hardware as used in the car.
GM's engineers are also collaborating with researchers at the University of Michigan through the Institute for Automotive Research. The U-M researchers are focusing more on basic combustion research including a lot of mathematical simulation. The GM staff are more involved in applying the work toward future production applications. GM engineers are also doing a lot of mathematical simulation work and the data from the engine test cells helps to validate the math models which ultimately speeds up the process.
During our visit to the Tech Center we also had an opportunity to drive the Saturn Aura HCCI prototype. We first drove this car almost two years ago at the GM proving ground in Milford. This time we actually got to sample it on public roads. The Aura's engine always starts in SI mode to warm the engine. A display on the dashboard shows the HCCI operating range based on engine speed and torque load along with a moving dot that shows where the engine is at any point in time. Before driving off we reset the timers that track how much time the engine was in HCCI mode. Putting the transmission in Drive with the brake applied puts the engine into the new mixed mode HCCI that was described above.
Driving off with a light foot on the throttle keeps the engine in HCCI mode although pressing harder will trigger a switch over to spark ignition. The mode change is largely transparent and is actually smoother than many hybrids. There is no shuddering shaking and you really can't feel the switch over. The only noticeable indication of a change is a slight momentary ringing sound reminiscent of diesel clatter. That clatter associated with older diesels is caused by the uneven explosive combustion that occurs in traditional diesels. That same non-homogeneous combustion can occur if all the parameters are not quite right during the start of HCCI combustion. It is typically resolved within only one or two combustion cycles, but the engineers are working to eliminate it entirely.
We drove up to speeds of 45 mph on Mound Rd in Warren and around the GM Tech Center and the engine behaved utterly normally. At the end of our 10 minute drive, the timer showed that the engine had been in HCCI mode about 75 percent of the time. While HCCI doesn't work well at higher speeds and loads such as highway cruising, GM doesn't see this as so much of a problem. Internal combustion cars are generally most efficient when cruising at highway speeds. The grind of urban stop and go is where such cars perform poorly and hybrids have their biggest advantage. This is also where HCCI engines can provide maximum benefit with fuel efficiency improvements in such conditions of over 20 percent. In the New European Driving cycle, the HCCI Aura achieves a 15 percent overall improvement in combined driving relative to a conventional spark ignition version of the same engine.
GM is also working on applying the same homogeneous charge concept to diesel engines and calling it pressure controlled compression ignition (PCCI). By ensuring that the air and fuel are completely mixed and using a pressure sensor to control the process, more efficient and cleaner combustion can result allowing diesels to operate with much lower NOx production. If this type of engine can be brought to market, it could further improve on the already efficient properties of diesels while lower costs by being able to eliminate much of the expensive after treatment systems.
According to Prof. Grebe, GM hopes to have the first production HCCI engines on the market in about five years. If the engine in the prototype made available for our drive is any indication, the company is well on its way.