As the industry plans its powertrain strategies for the mid- and long-term future, range-extended EVs are emerging as one potential solution.
General Motors made the first step with its Chevrolet Volt and its European cousin, the Opel/Vauxhall Ampera, which use a 1.4-L four-cylinder internal combustion engine as a range extender to power a generator for the car’s lithium-ion battery system.
Technologically effective though the GM solution may be, it is not low cost. If range extenders are to achieve high volumes and public acceptance, they must be cost-effective. Lotus Engineering believes a key element in achieving this will be the manufacture of small-capacity range-extenders that demonstrate realistic economies of scale, are package-efficient, and meet all expectations for NVH, durability, emissions, and fuel consumption.
To that end, the company has developed a relatively simple family of purpose-designed three-cylinder, and eventually twin-cylinder, gasoline engines. The aim is to supply the engines to OEMs with only minor modifications necessary for different applications. In fact the engines might be regarded simply as components.
“Component companies supply standard items like alternators and pumps to many manufacturers and it could be that the Lotus range extender engine could become just a component—built in very large numbers at a relatively low cost and used by a broad spectrum of OEMs across the world,” said Steve Doyle, Lotus Engineering’s Electric Vehicle Integration Chief Engineer.
To do so, Lotus needs to partner with a manufacturer capable of producing the engine in high volume. So it has been working with Fagor Ederlan to mature the original concept, optimizing performance and developing the engine for series production, explained Lee Jeffcoat, Lotus Engineering’s Chief Product Engineer – Efficient Performance.
A manufacturer of automotive components, Fagor Ederlan is part of the Mondragon co-operative. Mondragon’s broad production and business spectrum embraces refrigerators, washing machines, and even sporting guns, together with banking. Fagor Automatión, also part of the co-operative, is playing a significant role in this project, working on the generator and engine control system, Jeffcoat noted.
Because the Lotus engine is considerably less complex than a comparable automotive power unit, it does not need to be built by an engine maker per se, argues Doyle.
The Lotus engine has been developed as a triple. A twin-cylinder engine also is projected which would meet what Lotus regards as an increasingly strong market for outputs up to 25 kW (33.5 hp). The triple in prototype form has been used in the Jaguar XJ Limo Green program.
Both naturally-aspirated and supercharged versions have been designed by Lotus, with and without balance shafts. Turbocharging was considered but rejected mainly for packaging reasons. Lotus engineers decided a supercharger gives better packaging flexibility while retaining or exceeding the efficiency of a basic turbocharger. System cost of either approach is similar.
The Lotus triple has also been installed as a technology demonstrator in Lotus's Evora 414e sports car as well as in the Emas four-door concept from Proton, which owns Lotus. AEI recently drove the Emas on Lotus’ new Hethel, England, test track, which was constructed to meet FIA F1 standards.
Lotus’ work on the Emas involved design, build, and development of the complete series hybrid drivetrain, including systems integration and driving strategies, using extensive simulation modeling.
The range extender engine is placed under the hood, although it could theoretically be placed almost anywhere (it is mid-positioned in the Evora 414e). The motor and single-speed gearbox are under the luggage area floor. A single speed is deemed suitable for the Emas but Doyle stated that if an existing gearbox benefiting from economics of scale were available, a two- or even three-ratio version of that could be used to provide higher cruising/top speed.
The Emas has a 16.5-kW·h lithium-polymer battery with ten modules. The battery powers a liquid-cooled permanent magnet motor producing 75 kW (100.5 hp).
Claimed range is 55-60 km (34-37 mi) in pure electric mode. At that point the range extender kicks in to drive a generator that powers the electric motor and also recharges the battery. Like the Volt, it is still necessary to plug in the vehicle to achieve full charge while optimizing grid electricity's lower cost versus gasoline. A full charge on European household current takes eight hours, Lotus engineers claim. The system also features regenerative braking to help maintain minimum state of charge while underway.
"Genset" design approach
Doyle is fully aware that the downside to the range extender EV solution is the need for two energy sources, same as a regular hybrid. “But you can trade off the cost of the range extender with the cost of the batteries to achieve a sensible compromise for a battery EV,” stated Doyle. “The price of batteries is coming down but only slowly and to think otherwise (the so-called ‘battery breakthrough’) you’d be banking on something being invented that so far has not! So making a cheap engine is very much easier.”
Lotus started work on its Gen 1 range extender in 2009 and considered, depending on likely CO2 emissions, whether to base it around existing motorcycle or generator technology. “We also looked at Wankel and gas turbine technology and their respective packaging needs, “ says Doyle. Wankel-type range extenders are currently being tested by AVL and FEV, and Jaguar has developed a gas-turbine-based concept car.
Originally, Lotus's focus was on a twin-cylinder unit. A triple was chosen for initial production because packaging was acceptable (despite a relatively deep crankcase to accomodate a 95.5-mm stroke), and because the twin required a balance shaft which was deemed optional for the triple. Engine weight is 54 kg (119 lb) with the balance shaft, 51 kg (112 lb) without.
OEMs that have shown interest in the Lotus range extender currently are focused on engines capable of generating 35-50 kW (50-67 hp).
The 35-kW Lotus triple drives a liquid-cooled, permanent-magnet brushless generator. Engineers acknowledge its 41 kW (55 hp) continuous power rating is slightly more than strictly necessary.
In developing the engine family, Lotus engineers followed a “genset” philosophy, using the generator to start the engine with what they term “lots of safety systems and protocols” between the two.
A controller used on production Lotus cars has been developed for high volume manufacture to cope with the complex strategy necessary for pure electric or battery-plus-range-extender use.
Cost hurdles, OEM acceptance
Originally it was thought the range extender unit could run at a continuous 3500 rpm. This was changed to a 1500-3500 rpm spread which gave improved overall efficiency and also far less NVH in city traffic. Claimed specific fuel consumption is 240 g/kW·h, with burn varying little between the set engine speeds.
Direct fuel injection is not regarded as necessary by Lotus because the engine operates over a narrow rev band and there are no transients through the drive cycle. A throttle body is used to aid start-up and shutdown; in theory this could be eliminated. The engine now has a siamesed coolant jacket and an integrated inlet manifold.
The engine is installed at an inclined angle in the Emas, and vertically in the 414e. It also can be horizontally positioned for rear installation, a solution that Lotus engineers say is showing considerable interest. To offer such flexibility, the engines are designed to accomodate various oil pump and drain/scavenge configurations.
Full operating range of the Emas is about 560 km (348 mi). Top speed in battery EV mode is 169 km/h (105 mph). Top speed using the range extender drops to 96 km/h (60 mph); up to this velocity the charge is sustained but motor output drops to 35 kW. Lotus's CO2 target is 60 g/km based on the ECR101 test schedule. A supercharged version of the engine would provide a cruising speed of 120 km/h (75 mph).
The Lotus engine uses a close-coupled catalyst and has been designed to meet Euro6 and U.S. EPA Bin 2-3 emissions regulations, and is being developed to meet requirements into the 2020s.
Cost is the OEM's main concern with a range-extender EV project. With hybrid and EV volumes likely to remain low for some years, it is difficult to justify an all-new, purpose-built engine. That's one reason GM chose an off-the-shelf solution for Volt/Ampera. (GM engineers acknowledge the mass and packaging tradeoffs in using their Global Zero-family engine in the world's first production range-extended EV, but this was necessary to meet Volt's extremely aggressive development timetable—and deliver a 100% proven and reliable power system.)
Whether global OEMs would relinquish engine production to a supplier is another question. Combustion engine design, and full control of the production process, are considered core business to OEMs today.
If these concerns can be addressed to the industry's satisfaction, Lotus's range-extender approach could play a key role in future vehicle electrification.