Extended-range hybrid electric vehicles, also known as eREVs or plug-in HEVs (PHEVs), have a need for a modest-size onboard electrical generator (or genset) to recharge batteries that power the electric traction drive. The existence of this generating apparatus can easily boost electric vehicle range from 100 miles to 400 miles, removing range anxiety–a key restraint on battery electric vehicles (EVs), rendering them largely unacceptable for long-distance cross-country use. In some modern eREVs, such as the forthcoming 2011 Chevy Volt sedan, a small fuel-sipping I-4 gas engine is driving the generator. A small turbine genset was probably not considered for the Volt (due to cost), and probably not within GM’s realm of expertise.
An alternative patented genset prime mover has emerged from the UK by Bladon Jets Ltd.–a micro-size axial-flow gas turbine prime mover especially engineered for automotive use. The turbine will directly drive a high-speed alternator based on switched-reluctance (SR) technology; the system will rectify the high-frequency multi-phase alternating current (AC) output to direct current (DC) (a few hundred volts) to charge the lithium-ion battery pack. Bladon Jets claims that microturbine production costs will be less than internal combustion (IC) engines, in part due to simplified construction with BLISKS (1-piece integrally bladed disks). Technical Insights is skeptical of such unverified claims, in the absence of any mass production history. Bladon Jets is partnered with Jaguar Land Rover (Tata Motors) for vehicle integration and SR Drives (an Emerson Electric company) for the high-speed SR generators.
Figure 1 depicts the Bladon Jets micro gas turbine engine.
The project was funded by UK government’s Technology Strategy Board (courtesy of the Department for Business, Innovation, and Skills). The turbine size and weight savings over IC engines are considerable, and no cooling system or exhaust gas aftertreatment apparatus is needed. Micro-scale turbojets have been around for some time to power radio control (RC) scale-model aircraft, such as a B-52 bomber with eight tiny turbojets, but this is far different. See the picture, and adjacent pencil for scale. This little “screamer” surely packs lots of power for its weight, although the power capacity in kilowatt (kW) has not been disclosed for eREV use. Considerable muffling and sound isolation will be required. All simple-cycle Brayton heat engines/turbines exhibit low efficiency in converting fuel energy to work. That is a disadvantage, and explains why gas turbines failed as propulsion drivers for heavy trucks in highway use (though they are used in military heavy battle tanks, like the US M1AI Abrams). A recuperative turbine, recycling exhaust heat to inlet air with a heat exchanger, is somewhat more efficient but very bulky and adds cost/weight (it was tried with trucks, still the miles per gallon [mpg] was too low). For eREV use with microturbine gensets, no recuperator is anticipated.
The strategy driving the turbine-eREV R&D is reduced emissions (low carbon and carbon dioxide emitting, especially given the tough regulations in Europe). The reduced eREV weight will boost mpg. For eREV gensets, prime mover thermal efficiency may be secondary, given the limited expected run time over the test cycle. OEMs views on the efficiency issue will vary.
Frost & Sullivan – Technical Insight – June 2010