9.1 Engine Characteristics, Gear Selection & Transmission Control
Key Takeaways
- Peak engine torque and lowest specific fuel consumption occur in the same narrow rpm band, which manufacturers mark as the green sector on the rev counter.
- Gear ratios must overlap so that every change-up lands the engine in the top of the green band rather than below it, which is why block changing is correct practice in a heavy vehicle.
- Driving below the torque band makes the engine lug and stall under load; driving above it burns fuel and noise for power the road does not need.
- Gear hunting on an undulating gradient is corrected by manually restricting the transmission range, not by working the accelerator.
9.1 Engine Characteristics, Gear Selection & Transmission Control
Syllabus objective 1.1 of the Driver CPC is blunt about what a professional driver is expected to know: the characteristics of the transmission system, so that the best possible use can be made of it. Case studies rarely ask you to recite a torque figure. They put you in a scenario — a fully loaded double-decker on a 1-in-8 climb, an automatic coach hunting between gears on a motorway gradient, a driver who revs to the limiter out of every stop — and ask which response is professional. Every one of those answers comes from understanding where an engine actually does its work.
Torque, Power and Specific Fuel Consumption
A diesel bus or coach engine is described by three curves, all plotted against engine speed in revolutions per minute (rpm).
| Curve | What it measures | Where it peaks on a modern PCV diesel | Why the driver cares |
|---|---|---|---|
| Torque | Turning effort at the flywheel (Nm) | A broad plateau, typically from around 1,000 to 1,400 rpm | This is pulling power — it moves a laden vehicle away from a stop and up a gradient |
| Power | Rate of doing work (kW/bhp) | Much higher, typically 1,800 to 2,200 rpm | Power sustains high road speed, but reaching it costs fuel |
| Specific fuel consumption | Fuel burned per unit of work | Lowest in the same band as peak torque | The cheapest, quietest, cleanest rpm to drive in |
The three curves overlap in one crucial way: the engine speed at which torque is strongest is also the speed at which it burns the least fuel for the work done. Manufacturers mark this band on the rev counter, usually as a green sector, and it is often only 600 to 800 rpm wide.
The Green Band Rule
Driving inside the green band is not a fuel-saving nicety — it is the definition of competent PCV driving.
- Below the band the engine labours. Torque falls away, the vehicle strains, combustion is poor, and the driver compensates with more throttle. On a gradient this is how a laden double-decker stalls mid-junction.
- Above the band the engine is producing power the road does not need. Fuel consumption climbs steeply, cabin and saloon noise rises, and passenger comfort drops. Revving to the governor before every upshift can add a significant margin to fuel burn over a shift while making the ride worse.
- Inside the band the engine is at its most efficient and most flexible, with torque in reserve if the gradient steepens.
Gearbox Ratios and the Cover Diagram
A gearbox exists to keep the engine in that narrow band while road speed varies enormously. A gear-ratio cover diagram plots the road speed each gear delivers across the usable rpm range; the bars for adjacent gears must overlap, so that changing up drops the engine into the top of the band rather than below it.
[ GEAR COVER: ENGINE STAYS IN THE GREEN BAND ]
Gear 1 |====| (0 - 12 mph)
Gear 2 |======| (8 - 20 mph)
Gear 3 |========| (15 - 30 mph)
Gear 4 |==========| (25 - 45 mph)
Gear 5 |============| (38 - 62 mph)
overlap ^ ^ ^
Each change-up lands in the TOP of the green band, never below it.
- Block changing (1 - 3 - 5, or 5 - 3 on deceleration) is normal and correct in a heavy vehicle. Working sequentially through every ratio wastes time, wears the clutch, and produces exactly the jerky longitudinal movement that objective 1.5 tells you to eliminate for passenger comfort.
- Downshifting before a climb, while the vehicle still has momentum, keeps the engine in the torque band. Waiting until the vehicle has slowed and the engine is lugging forces a change under load, which is harsher on the driveline and on standing passengers.
- Never coast in neutral. Freewheeling downhill removes engine braking, leaves the service brakes to absorb all the energy, and is contrary to safe practice in The Official DVSA Guide to Driving Buses and Coaches.
Automatic and Automated Transmissions
Most modern buses and many coaches use a torque-converter automatic or an automated manual (AMT). The gearbox chooses ratios, but the driver still controls the outcome.
| Control | What it does | Correct professional use |
|---|---|---|
| Drive (D) | Full range of ratios available | Normal service driving |
| Hold / range restriction (2, 3, D2) | Prevents upshifting above a chosen ratio | Long descents, and to stop gear hunting on undulating roads |
| Kickdown | Forces a downshift for maximum acceleration | Rarely justified with passengers aboard; it maximises fuel burn and throws standing passengers |
| Eco / power mode switch | Changes shift points | Leave in eco for service work unless the gradient genuinely demands otherwise |
| Retarder lever | Applies driveline braking independent of the service brake | Use on descents to hold speed — see section 3.2 |
Gear hunting — the box shifting repeatedly up and down on a rolling gradient — is the classic automatic-transmission case-study trap. The professional answer is to restrict the range manually so the box holds one ratio, not to fight it with the throttle.
Anticipation Is a Transmission Skill
Reading the road 15 to 20 seconds ahead means you select the right ratio before you need it. A driver who sees a hill start on a gradient coming and is already in the correct gear, with the handbrake set and air pressure full, will move away smoothly. A driver who arrives unprepared will roll back, over-rev, or stall — in front of a full saloon.
Worked Scenario: The Loaded Double-Decker Climb
A 12-metre double-decker, laden with 85 passengers, approaches a 1-in-10 climb on a school run. The engine has a green band of 1,000 to 1,600 rpm.
- 200 metres before the gradient: the driver eases off the accelerator while the vehicle is still at speed, letting momentum carry the bus into the climb rather than accelerating at the foot of it.
- At the foot of the climb: road speed is 24 mph. The driver selects a ratio that places the engine near 1,500 rpm — high in the band, with room to fall as the gradient bites.
- Mid-climb: rpm decays towards 1,100. Torque is still strong, so no further change is needed. Had the driver started at 1,050 rpm, the engine would now be lugging below the band and a change under full load would be unavoidable.
- Crest: the driver lifts early, letting road speed stabilise rather than accelerating over the brow into an unseen hazard.
The exam answer is the driver who used momentum and pre-selected the ratio, not the driver who held full throttle and let the box hunt.
A coach with an automatic gearbox is repeatedly shifting up and down on an undulating A-road gradient, unsettling passengers. What is the correct professional response?
On a modern bus or coach diesel engine, where does the lowest specific fuel consumption occur relative to the torque and power curves?
Why is coasting a bus downhill in neutral unacceptable professional practice?