Infinitely Variable Geared Transmission

The D Drive is a geared continuously variable transmission. Most continuously variable transmissions use a belt running across two cones. However, a geared transmission can transfer greater torques or loads before slipping. Continuously variable transmissions are important for fuel efficiency because they allow the engine to always operate at its most efficient speed. Then the transmission changes the gearing ratio as needed to run the output shaft ( the wheels ) at the desired speed. If the D Drive can be developed to the point of automobile usage, it could mean a big step forward in vehicle fuel economy. Let us hope the D Drive helps us save on gas in the near future!



If you like emerging high tech solutions make sure to read tomorrow's post. I will be writing about Honda's U3-X, an amazing electric unicycle that makes the Segway look like Granny's walker.

Does Coasting in Neutral Save Gas?

If you are going to slow down for a turn or a stoplight ahead, should you put your transmission in neutral and coast along to a lower speed? Bobby Likis explains that with an automatic transmission, this will use more gas than leaving the car in gear. The reason is because while you are coasting in neutral, the engine is running in idle and receiving fuel to power it. However, if you are coasting in gear, the engine management computer detects that no power is needed and cuts off all fuel to the engine. Engine revolutions are maintained by drawing energy from the rotating wheels and drivetrain ... it is something like the way electric engines can recover energy while braking. Cutting off fuel flow means you will use less gas then letting the engine idle in neutral.

So to save on gas with an automatic transmission, leave the car in gear as you coast to lower speeds.

Magnetic Gears Reduce Friction

Friction between moving parts in the engine and transmission consumes a considerable percentage of all fuel used by a vehicle. Any technique for reducing this friction gives an increase in fuel economy. Here is a demonstration of two magnetic gears which use magnetic fields to transfer force instead of gear pins in physical contact. Avoiding metal on metal contact leads to much, much less friction in the system. Of course, generating magnetic fields strong enough to support the loads experienced by transmissions in contact with a couple of hundred horsepower engine is more than challenging. But it is always good to keep an eye out for possible future technologies that could help us save on gas.

Overdrive

In days gone by, cars had simpler transmissions with fewer gears, sometimes only three. It was possible to add a separate overdrive which connected between the transmission and the driveshaft. This overdrive could operate in two modes: one in which the output rotational speed was the same as the input speed, and an overdrive mode where the output speed was greater than the input speed, with corresponding decrease in output torque.

All modern transmissions have the equivalent of overdrive built into them. Whenever you are running at highway speed, you should always go into overdrive. Put your car into the highest gear. At highway speed, you don't need a lot of torque and a high gear ratio lets the engine turn over at the slowest possible speed for your traveling speed. That allows the engine to operate in a regime where it is more fuel efficient. So in highway cruise, go into overdrive to use the least gasoline for each mile!

Give Your Transmission A Rest

If you drive a manual transmission vehicle, you always shift into neutral when you are waiting for a light or at a stop sign. But if you have an automatic transmission you probably leave it in gear. Remember that the transmission's job is to let the engine run in a narrow range of speeds (where it is most powerful or efficient) while using different gear ratios to provide a wide range of vehicle speeds. But how does this allow you to stop your car while the engine is running? The engine is still turning over but the output speed is zero. This would mean that the transmission would have to provide a gear ratio of zero. In fact it does not.

The torque converter is what allows a vehicle with automatic transmission to stop in gear while the engine is still turning over. In a manual transmission, the clutch directly connects the flywheel to the input of the transmission and so both have to stop together. Torque converters use a fluid coupling system to connect the engine's flywheel and the input of the transmission. With a torque converter there is no direct mechanical contact. Because fluid is used to make the connection, the flywheel can be turning while the output of the torque converter remains stationary. When you are stopped at a light in gear, the engine is turning over but the torque converter passes only a very small torque on. This small torque can be easily resisted by the brakes.

Using your brakes while stopped in gear thus puts extra load on your engine. This extra load will result in using more gas. If you have an automatic transmission shift to neutral when you are stopped. Not only will it reduce engine load a little and save you some gas, but it will give your torque converter a chance to cool down.

Conventional Continuously Variable Transmissions

A few posts ago we talked about the NuVinci continuously variable transmission during our series on gas mileage technologies. The NuVinci design is an innovative type of continuously variable transmission that has not yet been used by a major automaker. However, there are other types of conventional continuously variable transmissions that have been used in cars. These are conventional in comparison with the NuVinci, but advanced compared to the common manual or automatic transmissions in almost all of our cars.

Everyday transmissions adjust the ratio between the rotational speed or RPM of the engine and the wheels by choosing from four or five fixed metal gears. Current continuously variable transmissions use a pair of variable diameter pulleys instead of the gears. Each pulley is formed out of two cones with their tips or apexes pointing towards each other. These cones can move closer together or farther apart. A hydraulic or spring system is used to control the spacing between the cones.

A belt runs between the pulleys, fitting in the groove between the two cones. Older designs used rubber belts that were shaped like a V to better contact the sides of the cones. Now with newer metal alloys there are metal belts. The metal belts are stronger and allow for transmissions that can support much higher torques. The idea is that as the cones move farther apart, the belt can slip farther down between them and get closer to the pulley axis. The lower down the groove the belt is the smaller the effective diameter of the pulley.

It is necessary to have variable diameter pulleys in pairs. If one pulley pushed the cones closer together forcing the belt up the groove and increasing the belt diameter the belt would have to stretch if the other pulley did not simultaneously decrease its diameter the same amount. One pulley of the pair is connected to the engine crankshaft and the other to the vehicle driveshaft. As they change their diameters in lockstep the ratio of crankshaft diameter to driveshaft diameter can vary continuously. The ratio of these diameters is in inverse proportion to the ratio of their rotational speeds or RPM. If for example the largest diameter (when the cones are close together) is double the smallest diameter (when the cones are far apart) then the output (driveshaft) RPM can vary continuously between one half the input (crankshaft) RPM and double the input RPM.

The result is better gas mileage due to two main factors. One is the fact that the engine can run closer to its most efficient RPM more of the time. The second is that the transmission is very simple so the losses due to friction within the mechanisms are reduced. These factors give a 6% increase in fuel economy.

NuVinci Continuously Variable Planetary Transmission

The transmission is the connection between the speed of the engine and the speed of the vehicle. Conventional transmissions provide a series of gears. Each gear has a fixed ratio between engine speed and vehicle speed. If you wish to go at a given speed and your transmission has for example five gears the engine has to run at one of five speeds. Each engine is most fuel efficient at exactly one specific RPM. If none of the five speeds is at that RPM, your engine will not be able to drive you at your chosen speed as efficiently as possible.

Continuously variable transmissions provide a range of ratios between engine RPM and driveshaft rotation (vehicle speed). This allows the engine to always run at its most fuel efficient RPM. Changing vehicle speed is accomplished by changing the transmission gearing ratio instead of the engine RPM.



There have been many implementations of continuous variable transmissions using a variety of techniques. Good old Wikipedia has a list of automobiles using them. Although automakers have only really been getting serious about their use in the last five years or so. Fallbrook Technologies has recently been developing a new type : the NuVinci. Watch the video above to see how it works. I think it is an ingenious mechanical system. This is a planetary continuously variable transmission. The name comes from Leonardo da Vinci, who first invented continuously variable transmissions 500 years ago. And Detroit has only seen the value about 5 years ago. Oh well, better late then never.