Planar optical waveguide

The compressed gas stored in the tank consists almost entirely of air because it is collected only during engine overrun. Therefore, injecting this gas into the exhaust manifold during full load acceleration affects the reading from the UEGO sensor, which ceases to be indicative of the in-cylinder air fuel ratio (see Figure 4.21a).

For both the conventional and the BREES-enabled operation, immediately after the tip-in the fuel mass flow was increased up to the smoke limit. During the BREES transient, the reading from the UEGO was affected by the additional oxygen from the air injection. A model-based correction would be necessary to estimate the air-fuel-ratio if it was required during this phase of operation.

22 24 26 28 30 0

5 10 15 20

Time (s)

UEGO (%)

Conventional BREES

(a) Exhaust gas oxygen

22 24 26 28 30

400 500 600 700 800 900

Time (s)

Exhaust Temperature (K)

(b) Exhaust gas temperature

5 10 15 20

−100

−50 0 50 100

Time (s)

Brake Torque (Nm)

(c) Engine braking

Figure 4.21: Secondary effects of the BREES system on engine/vehicle operation.

Data acquired during the tests indicated that the compressed air injection does not significantly affect the exhaust manifold gas temperature (see Figure 4.21b). A certain effect is, however, captured: it is observed that the initial build-up of the exhaust temperature in the case of BREES is slower than in the conventional mode. There are two potential reasons why only a small difference is observed. Firstly, the ratio of mass flow from the block is at least comparable to the air injection. Secondly, the temperature is measured with 1.5 mm K-type thermocouple, which has a poor bandwidth. The simulation environment is more suitable for such an investigation and results presented in the first section of this chapter (see Figure 4.4b) confirm the temperature of the gas flowing through the turbine is reduced from 1000 K to 800 K during the start of the transient

due to the addition of the cooler assist-gas. The increased mass flow through the engine into the exhaust quickly mitigates this effect. Nevertheless, compressed air injection affects the gas temperature for a short time and the impact of BREES on the durability of turbine should be assessed. The fact that the temperature does not actually drop, only its rate of increase is reduced, indicates that it is unlikely the turbine longevity would be significantly affected.

During the BREES-enabled operation, the low-end engine response is signifi-cantly improved. This means, however, that at low engine speeds the mechanical components of the engine (in particular the turbine, the dual mass flywheel, the crankshaft and others) are excited with loads of different magnitude and fre-quency characteristics than in the case of conventional operation. A detailed characterisation of these effects has not been pursued in this study.

An important challenge for the BREES system is the consistency of the engine performance. The presented results were generated with a 24 litre tank and a single braking manoeuvre charged the tank to a level that was sufficient for a single tip-in acceleration. The tank charging took only 2 seconds, but the clutch had to remain engaged during braking, which may be not consistent with individual driving practice. During acceleration the tank was discharged within 1 second. It is, however, observed that the most critical period is the first few hundreds milliseconds, when the mass flow is greatest and the injection time could be reduced.

It is also observed that moving the end-stop of the VGT to facilitate the tank charging changed the overrun characteristics of the engine (see Figure 4.21c). To ensure the consistency of the engine behaviour during overrun, one could adopt the same strategy of closing the VGT also for situations when the compressed air tank is already full. Another option could be to deliver a consistent vehicle be-haviour by modulating the demand from the braking system. This approach has been investigated for electric hybrid vehicles with the capability of regenerative braking. Braking systems that are able to compensate for varying powertrain torque are known to be costly.

If the tank was charged only during braking, one could imagine a possible lack of repeatability during manoeuvres involving multiple tip-ins and no braking.

However, charging may also be possible during low load operation. This action

would require an increase in fuelling quantity and change in VGT position. The control challenge here would be to make this imperceptible to the driver. One possible drawback of this strategy is that the proportion of combustion products stored in the tank would be significantly increased, which may be particularly important when considering condensation/freezing issues.

Further investigation is required to determine the most effective design of the compressed air tank. An excessive tank volume would lead to a packaging problem. On the other hand having a bigger tank or a tank with multiple cham-bers would allow for increased assistance capability. A constant pressure tank as considered in [Ma and Ma, 2010] is another design option.

The design of a control valve or valves is another challenge. Ideally the valve could be accurately adjusted independently of the pressure ratio across it, which can be significantly greater or less than 1. Another requirement on the valve design is that it seals in both directions as this is crucial for maintaining the pressurized air in the tank and not allowing combustion products into the tank during high load operation.