4.1.3 Humid Air Motor
The humid air motor (HAM) is a promising development in the area of NOx reduction [11][12]. The concept uses waste heat to achieve up to 100% charge air humidity. This is realised in a special humidifying column, which replaces the charge air cooler, Figure 3. Prior to entering the column, water is heated to around 80℃ using energy from cooling water and exhaust gases. The water is vaporised in the column using energy from the charge air, which enters at about 200℃.
Compared to direct water injection and fuel emulsion methods, more water can be introduced to the engine, ranging from 1 to 3 times the amount of fuel. The charge air temperature is higher in the HAM concept, but the water introduced will give sufficient thermal ballast to ensure that the temperature of the charge air after the compression stroke is unaffected. A reduction of NOx in the range of 50-70% with no or minor change in fuel consumption is obtainable.
The main reasons for the reduction of NOx emissions are the reduced temperature due to the increased heat capacity of the charge, and the reduced concentration of oxygen. The resulting combustion temperature profile appears to be more uniform and the prevention of 'hot spots' ensures that NOx levels are significantly reduced. Since it is the water vapour that is released into the combustion air, seawater can be used and distilled within the system. Minerals and salt deposits are not a problem as a bleed-off system provides a continuous feed of seawater.
4.2 Other developments in piston engines
4.2.1 Miller Cycle
This is a modification of the 4-stroke cycle whereby the valve timing is altered so that the air intake into the cylinder ceases before the piston reaches bottom dead centre. The expansion cools the air charge which subsequently reduces the process temperatures and hence NOx formation. The compression pressure and therefore the combustion pressure is also reduced resulting in less mechanical load on the engine parts. Practical application of this technique makes increased demands on the turbocharger and charge-air cooler. The only precondition is that the turbocharger must have a high efficiency (at least 70%), and a high compression ratio (at least 4 : 1).
The advantage of this is a lower charge air temperature and hence lower NOx levels with no affect on specific fuel consumption, compression ratio or valve timing. In experimental engine tests, reductions of 40% NOx emissions have been reported by Codan et al [14]. Problems that may arise are difficulties at part loads and combustion problems when using fuels requiring high ignition temperatures, increased soot formation, and starting difficulties.
4.2.2 Free Piston Engines
As the desire for increased efficiency stimulates the need for even higher maximum peak pressures, the mechanical loads on the engine increase accordingly. In a conventional reciprocating engine, these forces are limited by the structural integrity of the rotating elements involved. The free piston concept converts the linear reciprocating motion of the piston directly into some form of usable power. This is achieved either by producing hot, high pressure exhaust gas for use in a power turbine, or coupled directly to a hydraulic pump or linear alternator As a consequence, the unit is less constrained by the mechanical loads inherent with conventional reciprocating engines and thus higher peak pressures are possible which in turn lends itself to higher efficiencies.
Originally invented in 1928 by an Italian named Pescara [15],it was designed to act as a gasifier to produce high-pressure exhaust gas feeding a turbine, although the patent identified the potential for further applications. Later work focused on the Free Piston Engine Pump [16], the Stirling Free Piston Engine [17] and NASA's SPRE Linear Alternator [18]. The development of free piston technology effectively ground to a halt in the 1960's due to the intrinsic problem of controllability, however, with the use of modern sensing and control methods, it has now become a feasible solution to modern power requirements.
There are typically two configurations for the free piston engine, as a single displacer piston, or as two opposed-pistons coupled by a connecting rod, as illustrated in Figure 4. The integral problem is one of synchronisation and detonation timing. The existing technology used in conventional engines is applicable to the combustion chamber design and scavenging system. However, development of the control algorithm governing injection timing is required. As electronic fuel injection and engine management systems are developed, this degree of control will become possible. The inherent flexibility of the unit to optimise itself for a given operating profile, for example low NOx running, is a particularly attractive feature. The operational parameters such as stroke length and compression ratio can be varied through simple changes to the control algorithm.
With the development of modern linear motors, there is a potential for the development of a highly efficient, low maintenance, power generation unit. The free-piston engine has once again become a topic of interest in applications requiring flexible power generation solutions.
4.2.3 Steam Injected Diesel (STID) engine
In this concept, high pressure superheated steam (500℃, 180 bar) is injected into a diesel engine where it acts as a steam engine to produce additional output power. Cooling water and exhaust gas waste heat is used to generate the steam with the superheater being placed before the engine turbocharger. It has been calculated that this new type of engine can achieve a thermal efficiency of 58%, with a reduction in NOx emissions by a factor of 4. To achieve this level of performance very large quantities of water, in the region of 4 kg/k Whr, will be required to achieve a steam to fuel ratio of around 2.55 [19].
4.2.4 Fuel Injection Rate Shaping and Common Rail Injection System
With the development of electronic fuel injection [20], the ability to control the quantity and timing of the fuel injection into the cylinder has opened up enormous potential. Individual injection units may be set for a particular operating profile such as reduced fuel consumption or low NOx running, by altering the profile of the fuel delivered. Retardation of fuel injection has been considered as a method of NOx reduction, however it has been found that the associated rise in fuel consumption makes it unfeasible as a primary method of emissions control. The greatest potential of the electronic fuel injection lies in the ability to deliver fuel more efficiently by matching the quantity, pressure and timing of the delivery to the exact demand from the engine. In addition, the fuel injection unit can be accesses by PC based software to allow for manual changes in settings and perform diagnostic tests.
The advent of electronic fuel injection has also led the way for the implementation of the common rail fuel delivery system [21]. The benefits of the common rail system such as operational flexibility, load independent injection pressures, reduced emissions, less parasitic loads with higher efficiency and cheaper operation, make for a very attractive fuel delivery system. Indeed, some manufacturers have eliminated the need for a camshaft with both fuel injection and exhaust valve opening electronically operated.
