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TESTS on POWER CONSUMPTION of RELIANT ROBIN TRANSMISSION

1. Gearbox

Reliant Robin gearbox, with electric motor attached, was bench-tested away from vehicle. Power was calculated from measurement of electric motor current and voltage and known efficiency of motor.

The gearbox was not run under load, but it is thought from previous experiments with vehicles running under load that transmission losses do not vary greatly with load, being mainly caused by viscous drag, shear etc in the lubricant.

Power measurements were taken at full motor speed, 3470 rpm, with the gearboxdriving in neutral and all four gears. As losses were significant, the gearbox warmed up very quickly (2 or 3 minutes); all but the first measurements were therefore taken with the lubricant warm - ie about 35°C.

The ambient temperature was low - about 5°C - but the viscous heating is such that external temperature is probably not very important - ie the gearbox will warm up sufficiently to bring oil viscosity to a reasonable minimum generally regardless of
external temperature, within stated limits.

Note that the circumstances are rather different from those of a petrol or diesel engine where too much viscous drag makes it difficult to start an engine when cold. An electric motor, on the other hand, is guaranteed to start regardless of ambient temperature.

2.1 Measurements using Reliant's SAE 80 gear oil.

Power consumption in Neutral position (no gear selected) from cold.

Volts Amps Power (watts) RPM
12 8.9 96 780
24 12.3 266 1560
36 13.5 437 2340
48 16 691 3120

*All power calculations in these and subsequent measurements assume a motor efficiency of 90%.

After 2 minutes, power in Neutral fell to 596 W, and after 10 minutes to 496 W. Gearbox temperature rose from 7°C to 35°C (ambient temperature about 7°C).

2.2 Reliant SAE 80 Gear oil.

Power consumption in different gears - gearbox cold (c.8°C).

Motor running at 3470 r.p.m. on 46 volts

Gear 1st 2nd 3rd 4th
Amps 13.3 14.4 16.2 18.5
Watts 575 622 700 799

2.3 Reliant SAE 80 gear oil.

Power consumption against time. Top gear, 46V. 3460 r.p.m

Time (mins) 0 2 4 7 10 (Neutral) 41.4?
Amps 18.5 17 16.3 16.0 16.2 (11.5)
Watts 766 764 675 662 670  

Temperature when hot 36°C, Neutral 11.5A - 476W.

3.1 Hydrobio 46 oil (Total)

Power consumption in gears. Gearbox warm (30°C) 3475 rpm, 46 volts. Final temp. 35°C.

Gear 1st 2nd 3rd 4th Neutral
Amps 12.2 12.7 14.8 16.6 12.3
Watts 505 526 613 687 509

3.2 Hydrobio 46 oil

Power consumption against time. Top gear. 46V. 3470 r.p.m. Gearbox warm (30°C). Final temperature 36°C.

Time (mins) 0 2 4 6 8 10
Amps 16.5 16.0 15.5 15.5 15.6 15.8
Watts 683 662 642 642 646 654

4.1Total Quartz 9000 (SAE 5W - 40)

Power consumption in gears. Gearbox Warm (c. 30°C). 3465 r.p.m, 46 Volts. Final temp 35°C.

Gear 1st 2nd 3rd 4th Neutral
Amps 13.3 13.8 15.2 16.4 13.0
Watts 575 571 629 679 538

4.2 Total Quartz 9000

Power consumption against time. Top gear. 46V. 3460 r.p.m. Gearbox warm (31°C). Final temp 35°C.

Time (mins) 0 2 6 8
Amps 16.4 16.3 16.2 16.2
Watts 679 675 670 670

5.1 Effect of reduced quantity of lubricant

The gearbox was run with half the normal quantity of oil - ie. 0.5 litre instead of 1 litre and then run for a short time without any oil immersing the gears, leaving only the residue of oil left on the gears. The above measurements were repeated - as follows.

0.5 litres of Total Quartz 9000. Gearbox warm, c 31°C. Input shaft 3460 r.p.m., 46 Volts at motor.

Gear 1st 2nd 3rd 4th Neutral
Amps 12.7 13.2 15.0 16.1 12.5
Watts 526 547 621 667 517

5.2 Total Quartz 9000 on gears, but gearbox empty of lubricant.

Input shaft 3460 r.p.m., 46 volts at motor.

Gear 1st 2nd 3rd 4th Neutral
Amps 12.2 12.5 14.4 15.3 12
Watts 505 517 596 633 487

The above shows that losses in the gearbox are only slightly dependent on the level of oil in the box. Even when empty, losses are only about 7% less than when it is full (compare section 4.1 above).

The losses thus derive mainly from seals, bearings and gear-tooth friction rather than from churning of the oil as has been ofter suggested. This implies that oil mist lubrication, rather than oil bath, would not greatly cut the losses.

6.1 Back axle losses

The vehicle transmission was assembled complete - gearbox and back axle - and was then run off-load by jacking up the rear of the vehicle. This test, together with the gearbox tests, enabled the back axle losses to be separately estimated.

Gearbox oil: Hydro bio 46. Back axle oil: Reliant's standard SAE 90 gear oil (90/140).

6.2 Power consumption: Gearbox and axle cold (c. 7°C). 46.5 volts, 3375 r.p.m.

Gear 1st 2nd 3rd 4th
Current - Amps 27.3 37.1 49 57
Power - Watts 1143 1553 2050 2385

6.3 Gearbox and axle were run for 10 minutes until warm - Gearbox 28°C, axle 27°C. Power then re-measured 46.7 volts, 3400 rpm.

Gear 1st 2nd 3rd 4th Neutral
Current (Amps) 20.1 29 39.5 49.3 (14)
Power (Watts) 841 1214 1653 2063 (585)
Power minus g/box 336 688 1040 1376 (from 3.1)

Transmission ratio

12.53 6.62 4.26 3.23  
Estimated road speed (mph) 17.3 32.7 50.9 67.3  

7.1 Road Tests

The vehicle was given brief road tests to measure the total power consumption at up to about 30 mph, and also to estimate the tyre rolling resistance, so that all the principal losses could be obtained and cross-checked for consistency.

7.2 Total Vehicle Losses

In the time available it was difficult to measure the vehicle's running current accurately, however it appeared to be approximately 75A at 30 mph on a horizontal road, at 46 Volts, ie about 3.5 KW.

7.3 Rolling resistance

Low speed current of the transmission (after breakaway) jacked up in bottom gear was about 13 Amps.

Low speed current in bottom gear running on the road was about 26 Amps. This means that the contribution from the tyres was about 13 Amps. But it is known that tyre rolling resistance is about constant with speed - at least up to 30 mph - it may even fall as the tyres warm up.

At 30 mph tyre losses would therefore be about 13 x 46 ie - about 600 Watts. (EMF was about 46 Volts)

Assuming, to a first approximation, that remaining losses were due to air drag, power consumption at 30mph was made up approximately as follows:

Losses Power KW % of total
Gearbox 0.53 15
Back Axle 0.69 20
Tyres 0.82 17
Motor 0.35 10
Air resistance 1.33 38
Total 3.5 100


Note that Tyre losses of 820 Watts (0.82 KW) is close to the value expected for a vehicle weighing 580 Kg with tyre rolling resistance of 1%. (m x g x Resistance% x Speed) - 800 Watts.

Conclusions

Note that Tyre losses of 820 Watts (0.82 KW) is close to the value expected for a vehicle weighing 580 Kg with tyre rolling resistance of 1%. (m x g x Resistance% x Speed) - 800 Watts.

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