An inventor by the name of Tom Kasmer is claiming that his Hydristor (a contraction of "hydraulic transistor") will revolutionize the auto industry. What? OK, let's take a step back. Basically, Kasmer has invented a particularly efficient hydraulic motor (or a pump, depending on which way it's being driven). This could be used as a replacement for a stepped-gear or CVT transmission (typically referred to as a hydrostatic drive in the off-highway agricultural and construction world), or as an energy storage and release mechanism for hybrids.
With regards to its suitability for use as a power-transmission device, Kasmer states that his motor is nearly 95% efficient, where as traditional drivetrains are only 60% efficient. I have to take issue with both claims. Traditional drivetrains pass something more like 80-85% of the engine?s flywheel power if they?re properly matched to the vehicle, with approximately half of the loss coming from the transmission, and half from the rest of the system (propshafts, differentials, axle shafts, etc.). Additionally, the claim of 95% efficiency only covers Kasmer?s motor. Since a second unit would be needed to act as a pump, the ?transmission? efficiency drops to 90%. And if other drivetrain losses add 10%, well, then we?re right back to where we are with traditional systems. That pretty much aligns with what I?ve seen in the tractor world, where if anything, hydrostatic drives are slightly less efficient than geared drives. Now, mount the motors right on the hubs, and maybe then we?re making serious efficiency improvements.
On the positive side of things, hydrostatic drives are capable of transmitting far more torque than traditional CVTs. They?re also capable of being packaged in a wide variety of ways, making them suitable for FWD, RWD, and AWD applications. And the potential ?gearing? range with a hydrostatic drive can be many times that of a belt-type CVT. So there?s some potential here, which is why such drive systems continue to become more popular for those that move dirt for a living.
As far as use in a hybrid drive system goes, it?s certainly possible; after all, this was the basis for the Ford Tonka concept truck of a few years ago. A motor/pump assembly can be mounted parallel to the drivetrain, with the unit acting as a pump during decel. Hydraulic fluid is stored under extremely high pressure, and discharged back to the unit to drive it as a motor when acceleration is required. Ford and Eaton (the vendor) referred to this as Hydraulic Launch Assist (HLA) and claimed it was capable of generating 600 HP from nearly a dead stop; Kasmer claims that a 0-60 time of 3 seconds is possible. Both claims are probably possible with a big enough reservoir (which would be an monumental engineering task in itself) and strong enough drivetrain components. Interestingly enough, the military is interested in HLA for its ability to get big vehicles moving very quickly.
Overall, how do I feel about this? Well, to be honest, I?m a bit skeptical (which is simply the nature of most engineers). Hydraulics tend to be a pain in the ass, usually requiring constant maintenance. They?re finicky enough when used on vehicles that see professional attention every 50-100 hours, so I?m not sure just how well they?d work in a market where people want to maintain their vehicles only every 100,000 miles (if at all). I?m not so sure we?ll see this technology in a Prius-like vehicle any time soon. But in bigger SUVs or pickup trucks, such technology might be just the ticket due to user tolerance for maintenance, the inability of current CVTs to handle typical light-truck torque requirements, and the suitability of such technology for tasks like trailer-towing.