Intake Air Management for Diesel Engines
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Abstract: Managing the supply of air to the combustion chamber is an important process to ensure consistent and reliable performance of modern diesel engines. Air management encompasses all aspects that affect the quantity, composition, temperature, pressure, bulk motion and cleanliness of the combustion air at the start of the heat release period. Details of the intake system, cylinder head and valve train design, pressure boosting technology and charge dilution requirements are all important aspects of intake air management.
Managing the supply of air to the combustion chamber is a critical aspect of modern diesel engines and can impact emissions, performance and fuel economy. Combustion air management is the process that is used to ensure that the air supplied to the combustion chamber at all operating conditions meets a number of requirements including:
- a sufficient quantity of oxygen is available to ensure complete combustion,
- a sufficient amount of diluent (i.e., EGR) is present to control the combustion temperature,
- the temperature and pressure (density) of the charge air is controlled,
- suitable bulk motion and kinetic energy is imparted to the charge air in the cylinder to support the mixing of air, fuel and intermediate combustion products, and
- the size and concentration of impurities such as dust and dirt is acceptable.
In older engine designs that did not have to meet stringent exhaust emissions requirements, air management systems could be relatively straightforward. In some cases, it was sufficient to simply ensure that the air was clean, that the flow capacity of the intake system was adequate to ensure peak torque and power objectives were met and sufficient swirl was imparted to the air as it entered the combustion chamber to support the fuel injection system in the task of mixing of air and fuel. Typically, no active control of any intake side hardware was required. Even as many engines started to adopt turbochargers and other forms of intake air compression, it was sufficient to simply ensure a proper match between the engine and compressor.
Pressure to lower emissions while maintaining or improving other engine performance parameters required that the intake air properties be better controlled and matched to suit the engine operating condition. This required the introduction more hardware to control these intake air properties. For example, wastegate control on the turbocharger was introduced to enable improved intake air boosting at lower engine speeds and to limit turbine speeds at high engine speeds, valves were introduced to mix some exhaust gas (EGR) into the intake air at some engine operating conditions, turbocharger controls become more complicated to ensure that boost and EGR requirements could be met and higher and higher intake air pressures required that the higher intake air temperatures resulting from compression be limited. All of this added complexity required that more sophisticated control systems with sensors and sophisticated control algorithms be incorporated to ensure everything functions as expected.
This paper covers the basics of pressure charging—including turbochargers, superchargers and systems with multiple compressors—as well as turbocompounding and an introduction to intake manifold design. There are a number of additional important aspects of intake air management that will be discussed in separate papers. These include:
Charge Air Temperature Management. Managing the temperature of the air at the time of fuel injection in diesel engines is critical to ensure proper engine operation. There are two aspects of charge air temperature management:
If temperatures are too high, charge density can suffer and combustion temperatures can become too high. This can limit engine output and lead to increased exhaust emissions. If temperatures are too low, starting the engine at low temperatures can be problematic and/or emissions during engine warm-up can become excessive. Various pieces of engine hardware are commonly used to achieve proper charge air temperature. In boosted engines, charge air coolers are used to keep charge air temperatures from becoming too high, these can transfer heat from the charge air to the engine coolant, the ambient air or a separate lower temperature liquid. Ensuring sufficient charge air temperature for cold starting and to maintain it during warm-up can be achieved with glow plugs, electric grid heaters or flame-type aids.
- managing high air temperature by cooling in boosted diesel engines and
- managing low air temperature by heating to facilitate engine start-up and warm-up at low ambient temperatures.
Exhaust Gas Recirculation. Exhaust gas recirculation (EGR), the process of recirculating some of the exhaust gas back into the intake system, is an important technology that has allowed modern diesel engines to achieve very low engine out NOx emissions. As can be imagined, introducing relatively high temperature exhaust gas into the intake air can have significant impacts on the temperature and composition of the combustion air supplied to the combustion chamber. In order to ensure proper functioning of an engine with EGR, various hardware components, such as valves and coolers have to be introduced to control the flow, temperature and distribution of EGR supply and the resulting mixture with intake air. As well, turbocharger sizing and technology choices can also be affected and steps must be taken to ensure sufficient oxygen is still available for combustion and sufficient EGR flow is available at all engine operating conditions.
Crankcase Ventilation. Engines with closed crankcase ventilation systems vent gases from the crankcase into the intake air system to be recirculated into the engine. This recirculated blowby must be properly managed. Also, while the recirculated gases are filtered, a small amount of oil and particulate can still be introduced into the intake system and accumulate on critical air management components such as the compressor. Over time, if a sufficient accumulation of this material occurs, it can have a significant impact on the air management system performance.
Control of Flow into and out of the Combustion Chamber. From the intake manifold, the flow must be transferred to the cylinder. In four stroke engines, this is accomplished with a port located in the cylinder head with a poppet type valve to open and close the port. A different set of valve(s) controls the timing of the flow of exhaust gas out of the cylinder and into the exhaust port. Valve timing in four-stroke engines can be either fixed or variable.
In two stroke engines, ports in the cylinder liner located near the piston’s BDC location that are alternately covered and uncovered by the piston are commonly used to control intake flow. After combustion is complete, the burned gases from a two-stroke are expelled from the cylinder either through exhaust valves or a different set of exhaust ports located near the piston’s BDC position. The portion of the cycle available for expelling exhaust gases and admitting intake gases in two-strokes is relatively short. Generally, the intake gases must be pressurized in order to allow the incoming air to quickly fill the cylinder and scavenge it of exhaust gases.