Automotive Powertrain: Transmissions and Powertrain Matching Presentation

MMME3070 Automotive Powertrain Lecture 6 - Transmissions & Powertrain Matching

Professor Alasdair Cairns Faculty of Engineering

Office: Coates C48

Content

• Learning objectives
• Transmission fundamentals
• Principles of gears (revision)
• Road load losses
• Gear ratio optimisation
• Performance
• Fuel economy

Learning Objectives

This lecture provides a basic introduction to automotive transmission and powertrain matching At the end of this lecture students should:

• Understand the basic performance functions of the automotive transmission
• Understand the basics of road load loss analysis (i.e. the losses acting against vehicle motion)
• Understand the basics of gear ratio requirements and selection

Transmissions - Overview

Why do we need a gearbox in a road car?

3 BUGATTI

Transmissions - Practical Use

Seven practical reasons why a gearbox is required:

1. To increase tractive effort when the vehicle is moved from rest
2. To improve hill climbing or descending ability
3. To allow the engine to be operated near to peak torque during vehicle acceleration
4. To allow the engine to be operated near to peak power at the required maximum vehicle speed ("Vmax")
5. To allow the engine to be operated at the most efficient point for a given vehicle speed (within the gear ratios available)
6. To avoid engine stall at low vehicle speeds
7. To allow the vehicle to be easily driven in either forward or reverse direction

4

Transmissions - Mechanical Advantage (1)

P=TO where: P is power (W) Ț is torque (Nm) O is angular velocity (rad/s)

• Engine speed ranges are typically limited to 850-7000rpm
• Required wheel speeds are 0-1800rpm (road car) or 0-2500rpm (F1)
• When the speed is too low or the load too great the engine stalls . The primary function of the gearbox is therefore to maintain the optimum torque and engine speed for a given vehicle condition, within the engine speed range:

1. For a given engine power, by gearing down the wheel speed we can increase the available torque at the road wheel
2. By gearing up the wheel speed we can effectively widen the speed range available ....

4

Transmissions - Mechanical Advantage (2)

P = P 1 2 Driver (G1) Driven (G2) TO =TO 2

• Mechanical Advantage refers to an increase in torque or force that a mechanism achieves through power transmission
• Power is the product of force and velocity
• Example showing a 2.7:1 gear ratio
• If we assume negligible transmission losses:
• By reducing the speed by a factor of 2.7 the torque available is increased by 2.7
• Akin to a leverage effect

4

Transmissions - Cascade Diagram

7000 µ=0.95 6000 - Vehicle tractive force 1st Vehicle road load 1 Traction limit 5000 µ=0.7 Tractive Force [N] 4000 2nd 3000 - 3rd 2000 4th 5th 1000 u=0.1 0 0 25 50 75 100 125 150 175 200 225 250 Vehicle Velocity [km/h]

• Data for a current production C-class vehicle fitted with a 5-speed gearbox and 1.6 litre SI engine
• Such tractive effort curves are commonly used in industry to categorise the performance of the powertrain. The peaks in the curves usually correspond to peak engine torque
• The traction limit shows the grip limit of the tyres (based upon an assumed constant tyre friction coefficient of 0.95, which is well above the real world value)
• Most manufacturers will design to exceed the red line to give higher tractive force throughout the gears ......

Vehicle Road Load

F =- 1 PVC A 2 where: Fd = drag force p = fluid density v = fluid velocity Cd = coefficient of drag (0.25-0.45) A = projected frontal area . The vehicle road load is the sum of the effects of:

1. Air resistance: vehicle drag effects (skin friction + form drag), the forces of which increase in proportion to v2 and become dominant at higher speeds. It follows that the power loss is proportional to v3
2. Rolling resistance: due to losses between the tyres and the road . Ideal values of both can be calculated . In industry, the influence of road load is often carefully measured by producing coast-down curves under tightly controlled conditions (performed with the transmission in neutral, with strict rules for weight, tyre condition, weather etc.)

• Gradient resistance must also be considered where applicable

Tyre Rolling Resistance (1)

Factor Loss (%) Tyre-to-ground friction 5-10% Aerodynamic 3-5% Internal friction (internal air and tyre movement relative to the wheel) 1.5% Deformation losses due to the hysteresis of the rubber compound 85-95%

• Data shown for a road car tyre running at 80km/h (50mph) with zero camber and slip angles . Ref: Bastow et al., "Car Suspension & Handling", 4th Edition, SAE International, (2004) . The energy required to compress the tyre at the front is not fully recovered at the rear. Why? . The rubber exhibits viscoelastic properties and hence it recovers slowly when deformed
• High internal friction
• Irreversibility in the form of heat losses. . .

Tyre Rolling Resistance (2)

• During rolling, the centre of the pressure distribution moves forward in the direction of motion ..

Tyre Rolling Resistance (3)

axle -

• Consider a hard particle passing through the interface from right to left - initially the deformation of the tyre will add to the local pressure (the tyre acts like a spring being compressed)
• As the particle passes beyond the axle the local pressure is relieved (spring compression is relieved) but not all of the energy is recovered - some is irreversibly lost as heat during the compression process
• Akin to rubber suspension - some of the spring force is "dampened" on the rebound
• This causes an offset in pressure profile towards the front of the tyre
• Some materials exhibit less hysteresis (e.g. silica) whilst inflating the tyres to a high pressure reduces the degree of deformation (at the expense of contact patch area and handling!)

Tyre Rolling Resistance (4)

- 1 F N S S F. = - N = C_N r rr r Where: N = normal load (N) [i.e. The product of the total laden mass and gravimetric constant, g] s = pressure centre offset (m) [also referred to as the rolling friction coefficient] r = tyre radius (m) Crr = coefficient of rolling resistance . During rolling, the centre of the pressure distribution moves forward in the direction of motion . The arising moment must be overcome in order for motion to continue

Tyre Rolling Resistance (5)

Some key facts on tyre rolling resistance:

1. Values of Cpr for car tyres (road and race) are usually in the range of 0.01-0.025
2. Measurements of Cr, show that it is relatively independent of speed under the most commonly encountered road car conditions
3. This assumption may not be viable for all forms of motorsport Over the last few decades pressure has mounted on the automotive industry to reduce tyre rolling resistance

• Originally achieved using larger diameter and slightly thinner tyres with hard tread compounds (compromise in wet road grip)
• Also pressure to reduce tread depths (a worn tyre has a lower rolling resistance by ~5% per mm wear)
• However, in 1991 Michelin patented a process for incorporating silica in the rubber, which reduced rolling resistance by ~25% without compromise in tyre dimensions
• In 2001, Goodyear introduced the Biotred technique to replace some of the carbon and silica with a starch derivative while still obtaining similar Crr
• Such low rolling resistance tyres enable up to ~4% reduction in fuel consumption over the European drive cycle.

Content

• Learning objectives
• Transmission fundamentals
• Principles of gears (revision)
• Road load losses
• Gear ratio optimisation
• Performance
• Fuel economy

Ideal Gearing Requirements

Vehicle Condition Powertrain Requirement Maximum vehicle acceleration or traction Sustained high engine torque Maximum vehicle speed High gear (low ratio) at peak engine power Minimum vehicle fuel consumption Sustained operation near to the "island" of lowest BSFC on the engine speed-load map (highly engine design specific) . A compromise must always be made between the required engine condition and sustained operation near to this condition over a vehicle speed range · For a given vehicle speed, the following engine conditions are usually favourable to fuel economy

1. Naturally aspirated gasoline engine: moderate speed (e.g. 2000-3000rpm) and low-to-moderate load
2. Turbo-diesel: low speed (1000-1500rpm) and high load
3. Turbo-gasoline: low-to-moderate speed and moderate load

Theoretical Approach - Geometric Progression

2. Bottom gear ratio set for maximum traction e.g. 16:1 7000 6000 - Vehicle tractive force 1st f Vehicle road load 1 Traction limit 5000 3. Intermediate ratios set for geometric progression Tractive Force [N] 4000 2nd 3000 1. Top gear ratio set for desired Vmax e.g. 4:1 (including final drive ratio) -3rd 2000 4th 5th 1000 0 0 25 50 75 100 125 150 175 200 225 250 Vehicle Velocity [km/h]

• Geometric progression refers to all ratios varying by a common factor . For the above 5-speed gearbox the ratios would be 4:1, 5.66:1, 8:1, 11.33:1 & 16:1
• Common ratio of 1.415, in reality this is rarely implemented.

Case Study 1 - Honda S2000

180 250 160 200 140 120 150 Power [kw] 100 POWER BAND Torque [Nm] 80 100 60 -Power [kw] 40 -Torque [Nm] 50 20 0 0 0 2000 4000 6000 8000 10000 Engine Speed [rpm] . The S2000 is a good example of a high performance naturally aspirated SI engine

• The gearing of the vehicle is specified to best manipulate the power band

Case Study 1 - Honda S2000

Tractive Effort Curves (Honda S2000 AP1) 10000 9000 8000 7000 Tractive Force [N] 6000 5000 4000 3000 2000 1000 0 0 50 100 150 200 250 300 Vehicle Speed [km/h]

• The traction limit is based upon an assumed constant tyre friction coefficient of 0.7 (more realistic)
• The solid lines depict the manufacturer's chosen gear ratios
• The dashed lines demonstrate geometric progression . In reality, it can be seem that the mid ratios are often longer (lower ratio) to cover a wider velocity range and provide good acceleration profiles (e.g. 0-100kph near peak power in 2nd gear)

S2000 - Second Gear Comparison

Tractive Effort Curves (Honda S2000 AP1, 2nd Gear) 8000 180 250 -Power [kW] 7000 160 -Torque [Nm] 200 140 120 Tractive Force [N] 5000 4000 3000 2000 40 50 1000 20 0 0 0 0 2000 4000 6000 8000 10000 Vehicle Speed [km/h] Engine Speed [rpm] · For a high performance road car fitted with a naturally aspirated engine, the objective is to make 0- 100km/h (0-62mph) around peak engine power · Geometric progression does not allow this, a longer gear ratio is required • When thereafter dropping into third gear, the objective is to maintain the engine within the "effective" power band · Power band refers to the speed range between peak torque and peak power (as indicated by the two pink vertical lines on the RH figure), the term "effective" refers to satisfactorily near to peak torque for effective continued acceleration. 20 40 60 80 100 120 Power [kw] 150 100 80 100 60 Torque [Nm] 0 Power & Torque vs. Engine Speed (F20C) 6000