Closed-loop systems are possible in diesel engines.
Another major cost challenge in building today's clean diesels is the production tolerances of emission-related components. All modern light-duty diesels now utilize high-pressure common rail injection systems, and these require extremely tight tolerances in order for the system to perform properly. Tighter tolerances increase production costs, making diesels more expensive and thus more difficult to sell to the cash-crunched consumer. The diesel's superior fuel economy means little when both the price of fuel and the cost of the vehicle are higher than that of its gasoline counterpart.
Closed-loop combustion control can be used to decrease diesel engine production costs and improve drivability and emissions performance. "Closed-loop" means that information is gathered about what is taking place during the combustion event, and this data is then used to modify engine management system operation. In gasoline engines, this is typically accomplished using oxygen sensors that are located in the exhaust stream. In the case of diesel engines, however, cylinder pressure data can be viewed to gain an accurate view of the combustion process. This information can then be used to modify engine control module (ECM) outputs such as injection timing, injection quantity and EGR system operation, as well as enhancing diagnostic functions such as the misfire monitor.Pressure Sensing Glow Plugs
The idea of measuring cylinder pressure is a good one, but it creates a question of where to locate the sensor. Most diesel engines don't have a lot of extra room in the cylinder head for another hole to be drilled. This problem can be solved by integrating the pressure sensor into the glow plug assembly, which extends into the combustion chamber and is directly exposed to combustion pressure. The finished package could then be threaded into the same hole that once housed the glow plug alone, and makes it possible to retrofit some engines that weren't specifically designed to use this technology. Individual cylinders can each have their own sensor and thus could be monitored and tuned individually.The PSG is based on a standard glow plug, but is modified to allow the glow plug rod to move axially (inline) in the body of the assembly. As pressure increases in the combustion chamber, the rod is forced outward against the sensor element, which is made up of a number of micro-fused silicon strain gauges. The PSG sensor element relies on a phenomenon known as piezoresistivity, where pressure changes the resistance of the strain gauges. The strain gauges are built into a commonly used configuration known as a Wheatstone bridge, which is able to produce a signal that is proportional to cylinder pressure. This signal is then measured, compensated and amplified by an electronic signal conditioner, which is also built into the PSG assembly.
It is possible to use other methods such as piezoelectricity or fiber optics as sensing elements in pressure sensing glow plugs. According to Ron DeGroot, global strategy manager – engines of Sensata Technologies, piezoresistivity is a superior approach because "a piezoresistive sensor gives an absolute measurement with stable sensitivity and thereby allows an accurate measurement of pressure. It is accurate over a wide range and retains this accuracy over its long service life. It is robust and proven and thereby reliable and affordable."The Diesel Combustion Process
Conventional (heterogeneous mixture) diesel combustion plays out differently from what takes place in most gasoline (spark-ignited) engines. In port-injected gasoline engines, the combustion chamber is filled with a homogeneous air-fuel mixture, which is then compressed and ignited by a spark at a central point. The flame radiates away from the spark plug and across the combustion chamber, until the air-fuel mixture is completely burned.
In the case of conventional diesel engines, the combustion chamber is filled only with air and possibly EGR gases during the compression stroke. Since diesel engines use compression ratios of around 18:1 (much higher than gasoline engines, which are somewhere around 9:1), this air is highly compressed and heats up rapidly as the piston approaches TDC. The diesel injection system then begins injecting fuel into the superheated air, and combustion takes place in a number of steps. The air-fuel mixture is very rich in some areas, and very lean in others. Because the mixture is not evenly distributed, a flame front develops that tends to burn extremely hot. This flame front represents the area where the majority of the engine's NOx is formed. (See attached diesel cylinder pressure graph.)
1. Ignition Delay (A to B) – It is important to note that the diesel fuel does not ignite the moment that injection begins. During the ignition delay period, the fuel is absorbing heat as it is mixing with the turbulent air. It is desirable to have as short a delay period as possible, but this depends on a number of factors. A major factor is the cetane rating of the fuel, which is an indication of how easily the diesel fuel will ignite. While 40 is considered to be minimum, a higher cetane rating means that the diesel fuel will ignite more easily, leading to a shorter delay period. Long delay periods mean that when the fuel does start to burn, it does so extremely fast, leading to rapid pressure rise and knock. This can also lead to an increase in NOx emissions.
A strategy that helps limit ignition delay is the use of "pilot injection." The basic idea is that one (or even two) small shots of fuel are injected into the combustion chamber prior to the beginning of the main injection. This small amount of fuel vaporizes and ignites quickly, therefore warming up or "preconditioning" the combustion chamber. Then, when the main injection starts, combustion chamber temperatures have already risen to the point that it doesn't take long for this larger amount of fuel to begin burning as well.
2. Premix Burning (B to C) – The diesel fuel begins to burn starting at Point B. The fuel spray from the injector is made up of many different size droplets, and the smaller droplets are the ones that combine with air, vaporize, and burn first. As heat is released, the larger droplets also start burning and a rapid pressure rise takes place in the cylinder. Premix burning is heavily influenced by the ignition delay period. A short delay period allows for a smoother pressure rise during premix burning and minimizes combustion noise and emissions. Fuel continues to be injected throughout the premix burning stage.
3. Diffusion Burning (C to D) – Temperatures in the combustion chamber are high enough that fuel now combines with air and burns almost as soon as it is injected. Fuel continues to be injected up to Point D, which is where peak pressures occur in the cylinder. Note that peak pressure occurs after TDC (as the piston is descending in the cylinder).
4. After Burn (D to E) – Fuel injection has stopped, but any remaining fuel mixes with the air and burns.Cylinder Pressure vs. Temperature
As mentioned earlier, cylinder pressure measurement can provide a "window" into the events taking place inside a diesel combustion chamber. Pressure is directly proportional to temperature; in other words, as combustion temperature increases, cylinder pressure will also increase. Why is this important? In diesel engines, the biggest emission control concern is NOx, and NOx is formed when combustion temperatures rise to very high levels. At low temperatures, nitrogen is inert and does not combine with oxygen. However, as combustion temperatures rise through a certain range, nitrogen will react with oxygen to form NOx. If we can view combustion pressures in real time, this data can be used to modify injection timing and quantity to help limit combustion temperatures and in turn, NOx. Generating less NOx in the cylinder means that less has to be done to treat the exhaust after the fact. Aftertreatment (catalysts and other hardware used to treat exhaust gases) is expensive, so anything that can be done to limit NOx formation in-cylinder will pay big dividends.
Timing is Everything
When using closed-loop control in a diesel engine, a major point of reference during the combustion cycle is 50 percent heat release. If cylinder pressure is being measured, the point where 50 percent heat release occurs can be calculated and compared to the crank angle. Adjustments can then be made to the injection timing to bring this point in line with specifications. Compensation can also be made for varying cetane rating of diesel fuel, which is a major issue here in the United States. This kind of fine tuning can be done for each individual cylinder, making the engine run smoother and stronger over its service life than what would be possible with an open-loop management system.
Cylinder pressure sensing technology could also help pave the way for alternative combustion approaches such as HCCI (Homogeneous Charge Compression Ignition). HCCI has the potential for enabling a low NOx/low PM combustion, which is virtually impossible to achieve with conventional combustion. As emission control standards continue to tighten, this and other ideas will be implemented as engine manufacturers work to extend the life of the internal combustion engine.
Tony Martin is an assistant professor of automotive technology at the University of Alaska Southeast in Juneau, Alaska. He holds Canadian Interprovincial status as a Journeyman Heavy Duty Equipment Mechanic. He also has 18 ASE certifications, including CMAT, CMTT, L1 and L2.
