M.D. Bassett, J. Hall, D. Darkes, N.A.J. Fraser and M. Warth,     MAHLE Powertrain Ltd, UK

ABSTRACT

This paper presents an overview of the design and development of a range extender engine. Key attributes for the engine have been identified, these being minimum package volume, low weight, low cost, and good NVH. A description of the selection process for identifying the appropriate engine technology to satisfy these attributes is given. The resulting design is briefly outlined and the development and optimisation of the engine to meet its performance targets is described, along with a presentation of the resulting performance achieved. Finally, an assessment of the achievable CO2 emissions level is made, based upon real world usage patterns.

1 INTRODUCTION

The UK has set, in the 2008 Climate change Act (1), the target that the net UK carbon account for the year 2050 is at least 80 % lower than the 1990 baseline. The transport sector accounts for almost 24 % of the UK national CO2 emissions, of which cars and road haulage vehicles account for nearly 80 % (2). Thus, present automobile development efforts are keenly focused on measures to reduce the CO2 output.

The electric vehicle does not generate pollutants as it passes through inhabited areas, and can potentially rely on its energy being provided by a selection of renewable sources, making it the focus of much current interest. Despite continued developments in battery technology, the overall range of such a vehicle is limited. Furthermore, once the battery is depleted relatively long recharging times are currently required before the vehicle is available for use again. Cost and range capability, possibly coupled with ‘range anxiety of the driver’, are still viewed as barriers to the widespread adoption of electric vehicles (3-5).

There is increasing interest in combining the desirable features of the electric vehicle with the range freedom of a conventionally fuelled vehicle, leading to the investigation of the extended-range electric vehicle (E-REV). The E-REV is defined succinctly, by Tate et al. (6), as “A vehicle that functions as a full-performance battery electric vehicle when energy is available from an onboard rechargeable energy storage system (RESS) and having an auxiliary energy supply that is only engaged when the RESS energy is not available”. The addition of an auxiliary energy supply, which is typically a gasoline fuelled combustion engine combined with a generator, has the benefit of enabling the ultimate range of the vehicle to be limited only by the combined capacity of the battery and a gasoline tank for the engine. For long journeys, when the battery and fuel tank are both depleted, the driver can simply refuel the gasoline tank in a matter of minutes, in the same way that current vehicles are refuelled. However, it is desirable that for the majority of time the vehicle will operate in a purely electric only mode, from the battery, and that the user recharges the vehicle when it is not in use, e.g. over-night. Thus, the battery should be sized to cope with the majority of daily usage that the vehicle will encounter, and only rely on the range extender for infrequent, longer, journeys.

MAHLE has developed a range extender (REX) engine to identify the requirements, and challenges faced in the development, of the components for such future engines. The MAHLE range extender engine has been sized to be suitable for a typical C-segment passenger car. Bassett et al. (7) analysed fleet vehicle drivedata to identify the typical daily usage pattern of such passenger cars which enabled them to determine the requirements for the electrical components and the range extender engine. It was concluded that 30 kW mechanical power output was required from the range extender engine.

2 RANGE EXTENDER ENGINE CONCEPT SELECTION

A full evaluation of the different possible layouts for the REX engine was undertaken to assess the most suitable for the intended application. To assess the different possible engine configurations a selection matrix was produced to rank the concepts on their suitability to application within an E-REV. These parameters were grouped by engine function, design and production feasibility. The parameters of highest significance for an E-REV were considered to be package volume, cost, NVH, weight and reasonable efficiency. The highest priority was not assigned to efficiency, as UN/ECE regulation No 101, Annex 14 (8) and SAE J1711 (9) give weighting factors for the tail-pipe emissions of zero emission vehicle (ZEV) capable vehicles. These weighting factors reduce the declared mass of emissions as the electric operation only range of the vehicle increases, effectively reducing the significance of fuel converter efficiency on the declared CO2 figure for the E-REV.

Minimising the package and weight of the engine was deemed to be of paramount importance to maximise the applications that the range extender engine would fit into. A flexible installation capability is desirable to provide different installation locations to be utilised. NVH takes on particular significance within an E-REV due to the fact that the engine is not operating for large periods of time, and should not be noticeable over the electric-only operation.

Table 1 shows the list of engine concept layouts that were considered. Parametric CAD models were created, shown in Figure 1, allowing a package volume comparison to be made and a cost / weight model was also generated from benchmark engine and component data. The results of this analysis, compared to a baseline in-line 3-cylinder engine, are also shown in Table 1. Based upon the output from these assessments an engine layout of 2-cylinder in-line was chosen, due to it having the smallest package volume, lowest projected production costs and lightest estimated weight.

3 RANGE EXTENDER ENGINE DESIGN HIGHLIGHTS

The final REX engine design is shown in Figure 2 with the main external dimensions and summarises the key engine target parameters. During the detailed design process attention has been paid to minimising the engine package volume, weight and projected cost.

The initial concept aimed to minimise the overall length of the crankshaft and generator assembly through omitting the coupling between these two components. By bolting the generator rotor directly to the crankshaft palm the total length of this assembly was reduced significantly, as depicted in Figure 3. This solution also has cost benefits; as well as there being no coupling, no additional bearings are required for the rotor. The two bearing crankshaft layout also allows the addition of an internal flywheel, which may be required due to the low inertia of the generator rotor. To further reduce the length of the engine, the flywheel was positioned between the two cylinders.

Removing the need for a balancer shaft was viewed as being fundamental to creating a cranktrain with low series production cost. Normally for in-line 2-cylinder engines, one or two balancer shafts are required to compensate for the residual primary forces. Instead, a 180°-540° firing interval was chosen, while this solution has no primary out of balance forces there is a contra-rotating primary couple, similar to that of an inline 3-cylinder. As no external torque is transmitted from the engine/generator unit, it is planned to suspend the REX on relatively soft mounts to isolate it from the vehicle body.

The effects of block construction on cost, weight, bore spacing (and hence engine length), and NVH were assessed. The chosen solution uses a cast-in iron cylinder liner with an open deck structure. The cylinder block was designed to suit the high pressure die casting process. This method was chosen as the lowest cost option for high volume series production.

The REX unit differs to the engine in a passenger car in that it is only required to operate at a few discrete, steady-state, speed and load points which obviates much of the benefits achieved through variable valve timing systems. Detailed packaging studies of the valvetrain layout revealed that actuating the valve directly with the cam, via a bucket tappet, creates a minimum total height for the cylinder head and cover which is limited by the cam drive sprocket, as depicted in Figure 4. A two-valve-per-cylinder, wedge shaped combustion chamber model was created. The layout of the valvetrain, and efforts to reduce the overall height pushed the...