SUPERIOR DOWNSIZING
3.4 Advantages of combining direct injection and turbocharging in spark-ignition (SI) engines
3.4.4 Effect of direct injection on knock and abnormal combustion
naturally aspirated torque can be expected to increase due to an increase in compression ratio and volumetric efficiency under these conditions, again because of the reduction of charge temperature due to evaporation of the fuel in-cylinder. estimates for the improvements due to this over PFI vary, but a figure in the region of 5–6% improvement in steady-state, naturally aspirated torque is reasonable [60, 61]. a lower boost requirement and increased naturally aspirated torque both imply an improved throttle response.
3.4.4 Effect of direct injection on knock and abnormal combustion
as was outlined in section 3.3.4, knock is a major limitation in the performance of any pressure-charged engine. Many potential means of addressing the problem have been investigated, including approaches based on engine architecture already discussed [39–43] and some targeting manipulation
of the combustion process directly. These include applying cooled eGr at full load [62–66] and excess air [67, 68], the former of which appears to show the greatest potential from a knock-suppression viewpoint [69, 70].
Many of these combustion-based approaches will be discussed directly in later chapters, but from a historical perspective it is interesting to note that several were investigated by ricardo in early experiments and reported on in 1924 [71], for much of which a dedicated pressure-charged research engine was employed, and also that the effect of the specific heat capacity of any diluent was understood through the early development of supercharged aero engines [72].
However, the use of DI is in itself extremely beneficial in terms of knock suppression in turbocharged engines. This is because of the ability to target fuel introduction into the combustion chamber such that most of its heat of vaporization is removed from charge air and not from the engine structure itself. This has the effect of cooling the charge to a greater extent than can be achieved with PFI, so altering the pressure and temperature histories of the end gases to extend their induction time so that equation 3.1 is less likely to be satisfied during the time available for combustion. This concept bears some similarity to that of turboexpansion [39–42].
Gasoline, however, has a relatively low heat of vaporization (approximately 300 MJ/kg [73], depending on its exact composition) but the effect is still marked. Alcohol fuels promise even greater benefits in direct injection SI engines [74], which can be expected to be greater still in the case of pressure- charged units. Bromberg and Cohn [75] have performed calculations which clearly show the effective increase in octane index that can be expected due to DI of ethanol and methanol, both of which have a significantly higher heat of vaporization than gasoline (at 910 and 1160 MJ/kg, respectively [73]). These calculations suggest that DI of a very small volume percentage of alcohol (less than 20% of the total fuel volume) introduced at inlet value closing (IVC) can increase the knock resistance of a low-octane gasoline introduced via the intake ports markedly in a turbocharged DI engine. From this, one can see that even when operating a DIsI turbocharged engine on gasoline, a significant effect on the knock limit can be expected.
There is a secondary effect in conditions of high load and low engine speed due to the fact that the introduction of fuel can be delayed until post- IVC. as already discussed in section 3.4.3, in these conditions increasing the valve overlap will have an effect of increasing air mass flow through the engine and will shift the compressor operating point to a more efficient point.
This can only practically be supported by DI for reasons already discussed, but the effect on combustion is significant: the greater mass of air flowing through the combustion chamber during overlap (which has been heated less due to the improved adiabatic efficiency of the compressor at the rematched operating point) will (a) scavenge the hot residual gases from the chamber
(where the pressure differentials between the intake and exhaust ports permit) and (b) internally cool the combustion chamber and valves. Both of these benefit the knock resistance of the engine and permit the increase in low- speed torque reported for turbocharged DIsI engines [58], before the effect of evaporation and the octane rating of fuels is considered.
The magnitude of the effect of the heat of vaporization of a fuel is influenced by architectural considerations. In order to make the best use of the heat of vaporization, it is very important to achieve mixing of the fuel and the charge air that is as good and rapid as possible. This leads to the adoption of high tumble intake ports in DI engines in which the injectors are positioned under the intake port. However, as discussed above, this injector position is something of a historical peculiarity, since it was originally developed for
‘air-guided’ stratified combustion systems, permitting ready manipulation of the tumble ratio by a tumble flap at low mass air flow [52]. In one recent homogeneous charge engine employing downsizing as the primary route to fuel economy, the tumble flap has been deleted [76], although the underport injector position is not as readily changed since production lines have already been laid down for this configuration. For best air–fuel mixing, however, a central injector position, close to the spark plug, automatically aligns the injection nozzle and its spray in the high-velocity air stream at the top of the intake port. such an injector position, while utilized extensively for stratified operation in spray-guided lean-burn combustion systems which presently employ piezo injectors [77, 78], has been employed in second- generation homogeneous turbocharged combustion systems [79], and has been demonstrated to provide combustion advantages with solenoid injectors rather than piezo-activated ones [16]. at least one other research engine with this injector positioning and specifically targeting downsizing has been shown [80]. Lückert and co-workers [77] also point out that injection pressure is very important with respect to vaporization of the fuel, and this is an area of continual parallel development with injection pressures for non-piezo injectors increasing above 120 bar.
Coltman and co-workers [16] present a turbocharged DIsI engine in which operation at stoichiometric air–fuel ratio (l = 1) is possible to >4500 rpm at up to 20.5 bar BMeP, with a maximum enrichment to l = 0.9 at maximum power. In this particular engine, many factors contribute to this (including the adoption of a water-cooled exhaust manifold integrated into the cylinder head), but maximizing the latent effect is undoubtedly an important factor.
This has been achieved with close spacing of the solenoid injector to the spark plug and a process of optimization of the orientation of the six injection sprays by utilizing a newly developed injector targeting code. The effect of engine architecture on DIsI turbocharged engines will be returned to in section 3.6.