[News] New hydraulic hybrid transmission doubles MPG in city driving

[Moontanman]

New hydraulic hybrid transmission doubles MPG in city driving

 

March 2, 2009 Mechanical transmission of power using gears is very energy inefficient. The familiar automotive multi-speed gearbox and differential suffers from the friction losses that result in 20 – 30% of engine power being lost between a car's engine and the wheels. Many techniques are being developed to eliminate mechanical transmission including Wheel Motors and Hydraulic transmissions that we have seen being trialed in UPS delivery vans. Now in an innovative new approach, Scottish company Artemis Intelligent Power has developed a hydraulic hybrid transmission system it says can double a vehicle's MPG in city driving.

 

New hydraulic hybrid transmission doubles MPG in city driving

[CraigD]

I think the gizmag article’s statement “The familiar automotive multi-speed gearbox and differential suffers from the friction losses that result in 20 – 30% of engine power being lost between a car's engine and the wheels” is wrong.

 

Sources like this Wikicars article state that the difference between the power produced by most cars’ motors (eg: bhp “brake horse power”) and that produced by their wheels when measured on a wheel roller dynamometer is “5-15% lower”. This loss includes friction due to not only the gearbox and differential, but to various propeller (drive) shafts, joint, wheel bearings, and tires. This agrees with printed dynamometers test results I’ve read, which for the high-performance cars on which typically done, tend to show around a 5% loss.

 

The in-city driving efficiency improvement claimed for the system described in the article, Artemis’s “Digitial Displacement Hybrid Transmission”, is, I think, due mostly to its regenerative braking feature, which allows its two rear wheel driving pump/motors to be used as brakes, storing energy that would otherwise be lost as heat by conventional disk brakes in a fluid/compressed gas reservoir Artemis calls an “energy storage accumulator”, and also by its replacing the gas engine’s electric starter with a hydraulic one powered by the main system, allowing the engine to be turned off when not needed, rather than idling, a feature found in electric hybrids like the Toyota Prius.

 

Artemis looks to me to have an impressive system. It’s not in principle much different than what you’ll find on most professional-quality riding lawnmowers or forklifts, and earthmovers, but appears to spins much faster and more efficiently than these. A good rule-of-thumb measure of the efficiency of a hydraulic system is the size of its fluid cooler, and from their promotional schematics, Artemis’s looks to have a cooler smaller than some much lower-powered hydraulic drive vehicles I’ve seen.

 

Notice that the promotional literature is being a bit evasive about their system’s efficiency in all conditions, listing only a comparison of the test vehicle (a BMW 530i) with its stock drivetrain vs. their system in a few usage test scenarions. This shows an impressive improvement – 20.3 to 42.9 MPG - for the “European urban” rating, but a smaller improvement – 29.7 to 40.5 – on the “combined”, and, like a Prius and unlike conventional vehicles, a drop in ts combined vs. its urban rating. I’ve a suspicion its mileage at sustained highway speeds is about the same, or possibly slightly less, than for the stock BMW.