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01/03/2012
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After various historic attempts to launch electric mobility, the current global wave
of automobile electrification is touching fixed grounds within the industry and market
 After various historic attempts to launch electric mobility, automotive electrification is finally taking hold.
However, pure electric vehicles will not be sufficient to fulfil all demands and requirements for mobility as we currently know it. The most important problem in this respect is the range of electric vehicles, which is at least five times shorter than ICE driving range, while the battery recharge takes 5 to 200 times longer than refilling a petrol tank. So pure electric vehicles may not possess main stream capabilities yet, but they do fill up some interesting niches.
In order to guarantee the greater success and penetration of (longer range) electric vehicles as a mainstream product in the future, smart blends of traditional and electric propulsion are mandatory today. Such blends are required to avoid the range anxiety of limited-range electric vehicles – in effect, an internal combustion engine operating as a range extender that takes over to circumvent entire battery depletion and substantially extend the driving range. Shorter battery recharge times, in combination with higher battery energy densities, will be the major theme of improvements for decades to come and, as this happens, the role of the range extender will get smaller and might even be replaced entirely by another clean power source, such as hydrogen fuel cells or hydrogen-powered engines.
At DTI, we concentrate on smart, compact and cost-effective solutions for plug-in hybrid electric vehicle (PHEV) powertrains. The solutions feature a multi-power mode structure where shiftable transmission components play a crucial role. Our aim is to minimise the complexity and number of (expensive) components, such as electric motors and gear sets. One of the configurations actually consists of a single e-machine (30-40kW), a two-speed shiftable planetary gear set and some fixed gears. Furthermore, we adopt an ultra-compact V2 cylinder engine (50-60kW) with variable compression ratio (VCR), so as to enable high combustion power at minimal displacement volume. An image of this powertrain is shown on the opposite page. This simple, but effective, powertrain enables five operation modes:
• Two (power-shiftable) speed ratios in pure EV mode (for efficiency maximisation, see above)
• One speed ratio in pure ICE mode (for highway mode)
• One parallel hybrid mode (for power reserve in highway mode)
• One powersplit hybrid mode (for mode transitions or 'CVT' driving)
These five mode transitions are sufficient to enable both pure EV driving, as well as ultra-efficient ICE driving and smart combinations thereof. The ICE is only operated at higher vehicle speeds and usually in a quasi-static way, as of the single ratio highway mode. This means that engine speeds are changing, with no faster rate than the vehicle speed itself.
Higher ICE speeds and low dynamics enable a very effective exploitation of variable compression ratio techniques, such as those from Gomecsys (see Automotive Design, May/June issue,
page 22). Occasional deficits of ICE
torque response (if required at vehicle accelerations, for example) are compensated for by electric motor boost, until sufficient vehicle speed has been regained and engine power is sufficient to overcome driving resistance. In effect, the overall agility of the PHEV is primarily delivered by the electric power domain, whereas the ICE delivers sufficient driving range. In case of low battery charge and low-speed driving, occasionally the 'CVT' driving mode needs to be enabled. In this instance, the V2 engine can be kept at a relatively high speed, while vehicle and e-motor are at lower speed. Fuel is then used to propel the vehicle, while extra ICE power is delivered to keep the battery charge up. The amount of battery charging depends on the energy planning and the (low speed) driving conditions.
It is by innovations such as this that DTI hopes to make hybrid electric vehicles more and more mainstream.
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Author
Ian Adcock
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