The collaborative project ‘Ultra Boost for Economy’ (or ‘Ultraboost’) is a UK Technology Strategy Board-funded project
intended to demonstrated drive-cycle fuel economy improvements in the region of 35% through the aggressive ‘downsizing’ of a large capacity naturally-aspirated engine and without the use of hybridisation. The project partners are Jaguar Land Rover, Lotus, GE Precision, CD-adapco, Shell, the University of Bath, Imperial College London and the University of Leeds, and it started in September 2010 with a duration of three years.
Land Rover is the lead partner in the project, with responsibility for engine build, general procurement, engine-mounted charging system design and overall project management. Lotus is providing a dedicated engine management system (EMS), 1-D modelling and know-how on pressure-charged engines, together with support for engine testing, which will all be conducted at the University of Bath, where dedicated boosting and cooled exhaust gas recirculation (EGR) rigs will be used for initial testing with the demonstrator engine. GE Precision provides engine design and machining capabilities. CD-adapco are supporting with steady-state and transient CFD analysis in order to support port and intake system design. Shell are providing the test fuels and autoignition know-how, providing support to the University of Leeds who will develop their autoignition model to assist with the 1-D modelling. Imperial College are specifying the charging system, with support from Land Rover and Lotus; they will also test the selected components in order to characterise them accurately so that the 1-D model is as robust as possible.
The general target of achieving the same torque curve of the naturally-aspirated version of the 5.0 litre Land Rover AJ133 V8 engine was an inherent part of the project, along with the ability to provide driveability comparable with the Land Rover V6 diesel engine. An outline specification of a 2.0 litre pressure charged DISI engine was decided upon at the outset to the project. This represents 60% downsizing and requires operation at up to 32 bar brake mean effective pressure (BMEP). Downsizing helps to achieve strong reductions in drive-cycle and real-world fuel consumption through a combination of operating the engine at a higher load (for any given road condition) and a reduction in friction through, in this case, a significant reduction in the number of engine cylinders. However, an extremely challenging target for the project in matching the torque curve of the original V8 is to achieve 25 bar BMEP at 1000 rpm, where preignition and other abnormal combustion phenomena would normally be expected to be a severe limitation. Taken together, these targets are extremely aggressive and it can be seen why the project was deemed worthy of funding by the TSB.
The project is split into several parts. In Phase 1, which has already been completed, a 5.0 litre AJ133 V8 engine was commissioned on the test bed at the University of Bath, using the production Denso EMS. This was then replaced with the Lotus engine management system, which was shown to be easily capable of controlling the engine and giving exactly the same performance at full and part load. This phase therefore set fuel consumption benchmarks for the later downsized engine design and proved the capability of the Lotus EMS when controlling a direct injection engine with twin electro-hydraulic cam phasers and two high-pressure fuel pumps, while utilizing multiple-injection and multiple-ignition strategies.
In parallel with the Phase 1 engine test work, the Ultraboost Phase 2 engine was specified, designed and procured. In order to do this, the pooled knowledge of all the parties was used, resulting in a current industry best practice high-BMEP engine. The Phase 1 engine was designed to use the cylinder block a main bearings of the donor Land Rover V8 engine, with one cylinder bank blanked off. A completely new cylinder head was designed which incorporated twin cam phasers, high flow and tumble inlet ports (themselves the subject of much work on the part of the partners), a second-generation close-spaced direct injection combustion system, and cam profile switching (CPS) on both the inlet and exhaust cams. The use of CPS tappets on the inlet was in order to allow investigation of a degree of Miller-cycle operation both at low- and high-load, and their use on the exhaust side was in order to minimise pulse interaction at low engine speeds, which is an issue for cylinder groups comprising more than three cylinders, as is the case here. The 1-D model was developed in concert with this design process, and was used to help guide it.
The Phase 2 engine was fitted to the test bed and commissioned with no issues. Initial operation was at the naturally-aspirated condition, with the 1-D model being used to define exhaust back pressures and help to understand the best cam timing settings to use. This phase produced fuel economy figures at equivalent torque to the V8 engine which confirmed the possibility of reaching the 35% reduction in fuel economy for the non-boosted road-load conditions. After this test work, the University of Bath then commissioned their charging rig and this was used to increase the load and the engine was operated in the supercharged region.
Phase 3 of the project seeks to combine some minor modifications to the Phase 2 engine and to incorporate a self-contained charging system, allowing the engine to be used to gather full performance and economy data to prove the veracity of the approach. In order to do this, the 1-D model was extensively used to help specify the charging system, with several different technologies and technology combinations investigated. This work will be described in detail in a paper submitted to the upcoming Institution of Mechanical Engineers Turbochargers and Turbocharging Conference but, in summary, a turbocharger was selected as a low-pressure stage and a supercharger as a high pressure stage. Such a system has some similarities to that employed on the Volkswagen Group’s ‘Twincharger’ engines, but here the layout is different since the turbocharger is the first stage in the charge air path. This configuration was chosen for several reasons, including the ability to interpose an intercooler between the charging stages in addition to one conventionally-placed before the throttle. Conceptually, this is very similar to the charging system employed by the Lancia Delta S4 rally car, except that here the supercharger can be fully declutched, which in fact needs to be done above 3,000 rpm since it is being driven at very high speed to provide the necessary boost for driveability and response reasons below this engine speed.
In order to simplify the testing process, the 1-D model was also extensively used to set the intake conditions for the boost rig and the related exhaust back pressure so that, in the area where both charging devices work in series, so-called ‘supercharger-biased’ and ‘turbocharger-biased’ operating conditions could be investigated. This is important to investigate since, with an operating condition biased towards using the high-pressure supercharger to provide boost, one expects higher parasitic losses, but with a trade-off in potentially significantly better combustion efficiency (since there is a reduction in the mass of autoignition-promoting residual gases retained in the combustion chamber from one cycle to the next). This operating mode must be compared with achieving the same operating condition by closing the turbocharger wastegate and extracting more turbocharger compressor work (at the expense of increased residual rate due to the greater requirement for work from the turbocharger turbine increasing the back pressure at the exhaust ports).
Thus far during the Phase 2 testing, injection and cam timing have been investigated, together with PFI/DI fuelling split ratios (the engine also having port-fuel injectors in order to assess this), and cooled EGR has also been employed. Cooled EGR has shown significant benefits and the engine is easily achieving the initial high-load torque targets with extremely good specific fuel economy figures and virtually no preignition. However, in order to improve this further, a water-cooled exhaust manifold is also being procured and will be tested shortly.
An important extra dimension to the Phase 2 test programme will be to conduct some fuels testing. To date all testing has been conducted using a European-standard 95 RON gasoline; the test fuel matrix will permit investigation of RON and MON appetite for highly-downsized engines, which is a topic of significant interest for the industry at present; in attempting to understand this, Shell are at the forefront of the field, with mathematical treatment of the interaction of fuel RON and MON in relation to engine load having been undertaken for several years with an octane appetite weighting factor approach.
At the time of writing, the first of the Ultraboost Phase 2 engines had been removed from the test bed after more than 80 hours of very successful testing with no concerns (of which more than 15 were at high-load conditions above 20 bar BMEP, with most of these at or above 30 bar). It has been replaced with engine number 2 and will be stripped and inspected to gauge its mechanical integrity. The first Ultraboost Phase 3 engine fitted with the self-contained charging system described in the upcoming IMechE conference paper, is set to be tested from July 2012 onwards..
All of the partners express their sincere gratitude to the UK Technology Strategy Board for funding this exciting, challenging and successful engine technology project.
Author: Jamie Turner