Close-ratio transmission

A close-ratio transmission is a vehicle transmission in which the difference between the gear ratios of the gears is minimized relative to other transmissions used in similar vehicles. The use of "close" describes nearness rather than open-ness (i.e. open vs close).

There is no industry standardization between manufacturers as to what constitutes a close-ratio transmission. Therefore, a transmission that one manufacturer terms close-ratio may not necessarily be considered close-ratioed by another manufacturer.

Comparison with ordinary transmission

This table compares the ratios of two BMW 323i vehicles from the 1980s, the first being the series transmission and the second being the optional close-ratio transmission.

In the series transmission, the 1st gear ratio is 4.73 times that of the 5th gear ratio, while in the close-ratio transmission the 1st gear ratio is 3.76 times that of the 5th gear ratio. Given that both transmissions have 5 gears, this is indicative of the latter transmission having gear ratios spaced closer together; hence, its being a close-ratio transmission.

Mathematically, this closeness can be represented by the cumulative average spacing between, or geometric average of, gears. For the above series transmission, each successive gear's ratio is 68% of that of the preceding gear (e.g. (1 / 4.73)^1/4 = 0.678). The close-ratio transmission has successive gear ratios 72% of that of the preceding gear. Thus, the close-ratio transmission above has each successive gear "closer" in numerical ratio to the preceding gear than that of the series transmission.

There is no specific figure that is used to denote whether the steps between gears constitute a normal or close-ratio transmission. Often, manufacturers use this term when offering a standard manual transmission and an optional, sportier transmission, one with closer ratios than the other, such as BMW above did. But this close ratio transmission is not necessarily closer in ratios than another manufacturer's normal manual transmission.

Application

To maximize acceleration, the engine speed of an automobile must be kept near that where maximum power is developed. For this reason, vehicles utilize transmissions; as the vehicle's speed increases such that the engine speed exceeds the speed at which maximum power is developed, the driver shifts to a higher (numerically lower ratio) gear, which reduces engine speed and allows continued acceleration.

Internal combustion engines found in passenger automobiles are able to operate over a relatively wide range of speeds. A close-ratio type of transmission is designed to allow an engine to remain in a relatively narrow operating speed. Alternately, a wide-ratio transmission requires the engine to operate over a greater speed range, but requires less shifting and allows a wider range of output speeds. Close-ratio transmissions are generally offered in sports cars, in which the engine is tuned for maximum power in a narrow range of operating speeds and the driver can be expected to enjoy shifting often to keep the engine in its power band.

For example, the 2013 Honda Accord LX produces 185 bhp at 6,400 rpm and 245 Nm of torque at 3,900 rpm. This results in a power band of 2,500 rpm. Relatively speaking, the power band of this engine is essentially 50% of the engine's operating range from idle to maximum power.^[1]

The 1986 Honda 1.5 litre V6 "RA168E" engine installed in the 1988 McLaren MP4/4 Formula 1 racing car generated 750 bhp at 12,000 rpm and 664 Nm of torque at around 9,800 rpm. As a result, this engine had a power band of 2,200 rpm. The RA168E's minimum operating speed was 6,000 rpm. Relatively speaking, the power band of this engine was essentially 36% of the engine's operating range from idle to maximum power.^[2]

If we consider the Honda Accord above and suppose it is used with a transmission with the same ratios as the stock BMW 323i transmission above, then when the vehicle reaches 6,400 rpm in 2nd gear, shifting to 3rd gear reduces engine speed to 63.6% (1.40 divided by 2.20) of 6,400 rpm, or 4,070 rpm. Given that the Accord's maximum torque is developed at 3,900 rpm, this transmission would allow the Accord's engine to remain in its optimal power band.

Using these ratios with the Honda RA168E engine would result in the engine slowing to 7,630 rpm when shifting from 2nd gear at 12,000 rpm to 3rd gear. Given that the RA168E develops maximum torque above this speed, this transmission is ill suited for this engine, as the resulting engine speed is outside the engine's power band.

While the optional close-ratio transmission above is also not optimal for the RA168E application, it is an improvement. Shifting from 2nd gear at 12,000 rpm to 3rd gear reduces engine speed to 69% (1.61 divided by 2.33) of 12,000 rpm, or 8,290 rpm.

Close-ratio transmissions benefit passenger vehicles as well. The 2000 Honda S2000 produced 240 bhp at 8,300 rpm and 207 Nm of torque at 7,500 rpm, giving the S2000 a power band of 800 rpm, essentially 8% of the engine's operating speed range. Given the narrow power band of this engine, a close-ratio gearbox would be essential for maximum performance. The S2000's 6-speed transmission consisted of a 3.13:1 1st gear and 0.97:1 5th gear, resulting in the 1st gear ratio being 3.23 times that of the 5th gear ratio, vs 3.76 for the BMW 323i close-ratio transmission.

Close-ratio transmissions require a compromise, however. To maximize the dynamic performance of a vehicle, the automotive engineer would choose gear ratios that allow the vehicle to reach its theoretical maximum speed. For the BMW 323i above, that speed might be 130 mph given its 150 horsepower output at 6,000 rpm and aerodynamic and frictional drag at that speed. Thus, the engineer would select the appropriate rear axle ratio that, with the 1.00:1 5th gear ratio of the close-ratio transmission, causes the engine to turn at 6,000 rpm and generate the 150 HP needed at 130 mph.

This performance is useful in Germany, where vehicles regularly travel at their top speeds on the Autobahns. But in nearly all other countries, speed limits do not allow vehicles to travel at these speeds. Thus in top gear, the vehicles do not need to generate maximum power and engines can operate at speeds below those that generate maximum output. To increase fuel economy, decrease wear on the engine, and decrease noise, engine speeds are lowered by decreasing the 5th gear ratio. By using the BMW series transmission above, the 323i would gain the benefit of reducing engine speed in 5th gear to 81% of that resulting from the close-ratio transmission. But at 130 mph, the BMW 323i's engine speed would be 23% higher, or 7,400 rpm, well above the engine's maximum operating speed. Thus, with the series transmission, this vehicle would not achieve its theoretical maximum velocity.

Pseudo close-ratio transmissions

One way to create a close-ratio transmission is to install more gears into the transmission without altering the lowest and highest gear ratios. In this manner, some six-speed transmissions available in consumer vehicles are labelled as "close-ratio". Again, the defining issue is the overall spacing of gears between 1st and in this case 6th gear.

Whether a six-speed transmission can be legitimately called "close-ratio" depends on whether it keeps the top gear unchanged relative to that of a comparable 5-speed model, thus causing the change in ratios from low to high gear to occur in smaller steps (i.e. closer ratios) between gears. Alternatively, some six-speed transmissions have ratios essentially the same as a 5-speed transmission, and add an even higher (numerically lower) 6th gear that allows even lower engine speeds at highway speeds. In this case, the transmission would be considered a "double" overdrive transmission, depending upon the 5th and 6th gear ratios.^[3]

By extension, an automatic transmission could also be called close-ratioed. With the advent of 6-, 7-, and 8-speed automatic transmissions, the ratios become closer and closer together, which meets the mathematical conception of what constitutes a close-ratio transmission.

Continuously Variable Transmissions (CVT)

Prior to the 1970s, manufacturers' manual transmissions generally had three or four gears. To meet requirements to maximize fuel economy, manufacturers began offering 5- and, in the 1990s, 6- speed manual transmissions. Likewise, 3-speed automatic transmissions were the norm until fairly recently, but now 6-, 7-, and 8-speed automatic transmissions are being offered.

By reducing the spacing between ratios allowed by having more gears, a vehicle's engine speed can be kept in a narrow band. With a 5-speed transmission, the power range must be relatively wide, which requires compromising the engine's efficiency. With an 8-speed transmission, the power range can be kept relatively narrow, which allows the engineer to optimize engine efficiency at a particular engine speed, and the transmission attempts to keep the engine operating at that speed.

(Engine efficiency improves greatly when the load on the engine is maximized; hence, automatic transmissions also upshift whenever possible in an attempt to lower than engine speed as much as possible, which increases load and efficiency.)

The recent introduction of continuously variable transmissions (CVTs) attempts to push this strategy to its logical conclusion. This allows a near infinite "number" of gear ratios, which this implies an infinitely close-ratioed transmission. However, given that there are no gears or specific gear ratios, one would not really consider such a transmission close-ratioed.

References