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Interesting Hybrid Technologies

The coolant thermos! You’ll find this in the 2004-2009 Prius. It is exactly what it sounds like; a insulated container that holds coolant and keeps it warm just like thermos you’d put coffee in. Why would you want to keep your engine coolant warm you ask? A warm engine operates much more efficiently than a cold engine. When the engine is warm the pistons have expanded to their operating size, the piston to cylinder wall clearance is optimal and blow by (think wasted power sneaking past the piston) is reduced. A warm engine will also use less fuel because fuel atomizes (mists) better in warmer temperatures. Only atomized can be burned by the engine. The more of the fuel that atomizes, the less fuel is required. Auto makers have been trying to get there engines to warm up quicker for years. They’ve prevented the transmission from going into overdrive until the engine is warm to keep the RPM up for a faster warm up, reduced the coolant volume, … The thermos is by far is the best idea yet. You’re likely to see this idea creeping into standard gas cars in the future. The cost to add a thermos must be pretty low and the improvement in fuel economy and emission will be great.

The fake brake! When you step on your brake pedal in a conventional system, you’re pushing a piston that is pushing fluid out of a cylinder (think bicycle pump), through brake lines, to a hydraulic caliper that squeezes stationary brake pads against a rotor that spins along with the wheel. When you step on the brake in a Prius, you’re still pushing fluid with a piston, but instead of going to the calipers, the fluid goes to a “dead end” chamber with a piston that pushes on one or two springs. The only purpose of the chamber is to provide a pedal feel for the driver. When the brake is pressed with moderate pressure, the piston is only touches the softer of the 2 springs, providing a little give in the brake pedal. This simulates line and caliper flex normally felt on a conventional system. When brake pedal is pressed harder, the piston comes in contact with the second, stiffer, spring, providing a harder brake pedal, simulating the feel of a conventional brake system once the “slack” has been removed. The real braking work is controlled by a brake control computer, which monitors the brake pedal position with two redundant sensors. The brake computer then uses the electric motor / generator MG2 to slow the car and charge the battery or applies pressure to the calipers using pressure stored in the ABS accumulator. If there were a serious brake control problem, two solenoids open, by-passing the dead end chamber and computer control, allowing the brake system to function just like a conventional system.

Fly by wire!

Well, not exactly… When airplanes first abandoned lever and cable control of their rudders and such in favor of sensor and servo control, they called it “flying by wire”, referring to the electrical wires connecting the sensors and servos. The hybrids have abandoned the physical cable connection between the gas pedal and the throttle valve in favor of a gas pedal position sensor and throttle valve control servo. The computer needs to be able to control the internal combustion engine’s output, so your direct input is no longer required. Instead the computer observes how far down you press the gas pedal and then decides how best to provide the acceleration you’ve requested through the electric motor, the internal combustion engine, or a combination of both.

The single planetary transmission!

Anyone who has ever rebuilt a traditional automatic transmission will tell you, there are a LOT of moving parts to wear and fail. The transmission on the Prius has only one planetary gear set consisting of one sun gear, one ring gear, 4 planetary gears mounted to a carrier and a couple shaft bearings. That sentence accounts for all the moving parts if you don’t count the integral differential and drive chain. Mentioning all of the parts in a conventional automatic would take me pages instead of a sentence. I would guess that is has less than a quarter of the wearing part of a conventional automatic. It’s too early to tell how long these transmissions will last before needing repair, but it seems logical that the likelihood of failure goes down with decrease of parts that have the potential to fail. The power flow through the transmission is very simple compared to other automatic transmissions, but still requires putting on your thinking cap to understand it. Graham Davies has done a much better job of explaining how the single planetary transmission works than I ever could.

Electronic assisted steering

Most cars provide the driver with some form assistance when turning the wheels. Usually, this is done with a hydraulic assist system. A hydraulic pump is spun by with a rubber belt connected to the engine’s crankshaft pulley to build hydraulic pressure. The hydraulic pressure is directed to push the steering rack gear to the left or the right depending on which way you turn the wheel. There are two problems with this system. 1) the pump is putting a load on the engine whenever the engine is running and burning up gas whether you need help steering at that moment or not. And 2) the pump is not spinning when the engine is not spinning, which is a problem for hybrid vehicles, since the engine shuts off at idle. EPS (electronic power steering) is made easier to implement on hybrid vehicles because there is plenty of electrical power available.

 

Modified VTEC for Honda Hybrids!

Honda has used their VTEC system to modify valve timing for peak power and efficiency at different engine RPMs for years. Now they are using their VTEC system for a different purpose: reducing engine drag during deceleration so more energy can be captured with the electric motor/generator (AKA “MG”) to charge the battery pack. Honda, unlike Toyota, has the MG bolted directly to the crankshaft, so any time the gas engine is spinning, the MG is is also spinning and vice-versa. This creates a problem when trying recapture energy on deceleration. What Honda wants to do is spin the MG with the wheels as the car is slowing, but since the the engine is attached to the MG, some of the energy will be used up by the gas engine’s “compression braking”. Honda’s hybrid VTEC system solves this problem by keeping the intake and exhaust valves closed, preventing the engine from wasting energy by pumping air.

 

NiMH batteries!

NiMH batteries are great! We have been using them to power most battery powered devices in our shop for years. We haven’t had one fail yet despite poor charging practices, dropping them on the ground, and generally abusing them. Currently, all hybrids are using NiMH battery pack. The batteries should last a long time because the hybrid’s computer maintains the battery pack in a way that will provide maximum service life. The battery pack is never charged over 80% of full charge and charging does not begin until 40% of full charge. The hybrid’s computer also monitors the battery pack’s temperature and will shut off the electric motor assist if the battery pack begins to overheat.

Batteries can be rated by their energy to weight ratio, which is a very important metric if you are hauling your battery around with you. This type of rating is expressed as watt-hours per kilogram (Wh/kg). The higher the number in the Wh/kg rating, the more power the battery has per pound. Lead acid batteries (like the one you will find in a conventional car) have a Wh/kg rating of 30 Wh/kg. Ni-Cad batteries (the older style rechargeable that turned a lot of people off to rechargeable batteries) have a rating of 60 Wh/kg. NiMH batteries have a Wh/kg rating of 70 Wh/kg and have a much longer service life. Lithium-ion batteries have double the Wh/kg rating of NiMH at 140 Wh/kg, and are found in some full electric vehicles. I don’t know why they aren’t used in hybrids, perhaps the occasional news item about exploding laptops or cell phones has something to do with it, maybe they just don’t last long enough, or maybe they are hard to recycle or dispose of. If you know, drop me a line; I’m curious.

NiMH batteries can be recycled, much like the lead acid batteries are now, so they do not pose the disposal danger that some have claimed. In fact, the regular single use Alkaline batteries that everyone seems to be buying in bulk and throwing in the landfill pose a greater environmental risk than the long lived NiMH. For more on our battery recycling program, click here.