SUPERIOR DOWNSIZING
3.3 Problems and challenges associated with turbocharging the spark-ignition (SI) engine
3.3.3 Turbocharger lag
The delay in response between a demand for a load change and it being delivered can be very noticeable in turbocharged engines and is referred to as turbocharger lag (or more commonly ‘turbo lag’). It is a function of many factors and is being addressed in a number of ways.
any discussion of turbo lag needs to be conducted in the context of what occurs in both naturally aspirated and supercharged engines. In a naturally aspirated engine with conventional throttle placement, i.e. upstream of the plenum entry, a load step has to increase the density of the air in the plenum to close to that of the ambient via filling it with fresh air, and the volume of the plenum consequently effects the driveability of the engine. In engines with fixed valve timing and significant valve overlap, the plenum will contain a high proportion of exhaust gas at part load due to the low pressure level relative to the exhaust system, and a load step may therefore cause a slight stumble. This being the case, it is not unusual to instead specify smaller throttles in the intake port of each cylinder to minimize the throttled volume and so alleviate the exhaust gas carryover effect. This is common in high- performance engines.
supercharged engines also suffer lag. In a typical positive-displacement supercharger application (such as a roots- or Lysholm-type) the throttle is conventionally upstream of the supercharger. This means that a load step has to provide enough air to pressurize the entire intake system volume (including the intercooler) before the change in engine load is sensed. Furthermore, in the case of high boost supercharging, such a load step demands a significant instantaneous input in work into the supercharger, which is taken from the crankshaft. Hence, a stumble can manifest itself in such engines, too, which can be increased if the supercharger is clutched (and bypassed) and possesses significant inertia (compounded by the gearing of the supercharger relative to the crankshaft).
The turbocharged engine suffers from lag due to various elements of the above and to additional factors. Its general throttling is similar to that of the naturally aspirated engine insofar as throttling at the plenum entry and downstream of the compressor outlet is the norm (although port throttles might equally well be used). This mismatch in the supply and demand of boost pressure is possible compared with a supercharged engine because the turbocharger shaft is independent of the crankshaft, and hence its pressure ratio and mass flow can vary because there is no mechanical link between the two. There is therefore an initial response to a load step which is the basic naturally aspirated response of the engine (modified by the restriction
of the extra components in the intake system such as any intercooler fitted).
once this naturally aspirated level has been reached the lag is a result of the aerodynamic characteristics of the compressor and turbine (and how they interact with the positive-displacement reciprocating engine and its valve timing) and the inertia of the turbocharger wheel assembly, which is dominated by that of the turbine wheel (or exducer).
Figure 3.4 indicates the difference between the full-load steady-state torque curve and the torque generated transiently as a vehicle is launched. at very low engine speeds a low level of boost is produced and the performance is essentially that of a naturally aspirated (na) engine operating at part-load.
When the throttle is opened the engine very quickly reaches the full-load na level. The two curves then diverge as the dynamics of the charging system prevent the engine from reaching the steady-state curve. Because the engine is wastegated the two curves converge again because the steady-state curve is artificially capped.
Consequently, there is some advantage in not specifying an engine with a requirement for a very high boost pressure to be provided by a single compression stage, and this pragmatic approach is one means of mitigating turbo lag [16]. If, however, high pressure ratios are required, compound charging becomes more attractive. In a compound-charged engine, at least two distinct compressor stages work in a complementary manner to provide sufficient air to the engine across its full speed and load range. These systems usually take the form of a turbocharger coupled with (a) a positive
3.4 Turbocharged engine transient and steady-state torque curves.
Engine torque
Dynamic torque curve Steady-state full-load curve
Engine speed
displacement compressor [13, 15], (b) an aerodynamic compressor [12] or (c) another turbocharger compressor. This last configuration can have the compressors configured in either sequential [18] or parallel [20] configurations.
The ‘regulated two-stage’ approach was successfully demonstrated by Volvo in a methanol-burning engine in 1990 [21] and has more recently been successfully developed for production diesel engines [22, 23], whereas to date in SI engines the parallel configuration has most commonly been used in production [20]. The BMW diesel configuration is shown in Fig. 3.5. In any road-biased systems the aim is to provide improved driveability with better high-speed fuel consumption by rematching of the turbine stage as a consequence of the reduced need for it to provide all the driveability necessary [15]. Thus, the supercharger, or small ‘high-pressure’ turbocharger, provides the boost at low engine speeds, and a larger than usual, lower expansion ratio turbine drives the main compressor, providing the bulk of the boost at higher engine speeds and loads. Given its success in light-duty diesel production applications, it is expected that the regulated two-stage approach will be adopted for production gasoline engines at some point in the future.
In a historical context, the driveability of single-stage turbocharged engines has improved significantly over the years through a combination of engine control system development, the application of torque-based engine control, and improved understanding of the gas dynamics of all the devices in the charge
3.5 BMW ‘regulated 2-stage’ turbocharging system from the BMW 535d. (coutesy: BMW AG).
air path as well as the significant improvement in turbocharger technology.
over the same period of time, naturally aspirated engine performance has been improving due to widespread adoption of camshaft phasing devices.
such devices can also be applied to turbocharged engines and used to shift the operating point in the compressor map and so increase mass flow (with its beneficial effect on turbine work).
A further important operational benefit of using camshaft phasing devices is that the valve overlap can be reduced at part load, thereby minimizing the amount of trapped residual gas in the cylinder. The overlap can be increased as the boost pressure builds up, so that the engine scavenging and turbine mass flow rate benefit from the positive pressure differential across the engine, cooling the combustion chamber surfaces directly. However, with port fuel injection this would result in significantly increased amounts of unburned charge flowing into the exhaust manifold with its consequent combustion and overheating of exhaust system components. Hence camshaft phasing has not been applied as readily to turbocharged engines. With the advent of direct injection systems, however, there is now a means to prevent the introduction of fuel into the air until after the exhaust valve has closed and hence to provide a means to influence engine operation by compressor map shifting and to improve driveability further. The benefits of this will be returned to in section 3.4 below.