Mercedes Mullins
The fuel efficiency of a car is a topic that has been widely studied, with drivers keen to reduce their fuel consumption and maximise efficiency. In conventional vehicles, only about 12–30% of the energy from fuel is used to move the car forward, with the rest lost to engine and driveline inefficiencies. The amount of energy used to accelerate a car depends on a variety of factors, including the type of vehicle, the speed, the driving conditions, and the driving technique. For example, in city driving, a vehicle spends a significant amount of time idling in stop-and-go traffic, using energy to run the engine and power accessories. In contrast, highway driving includes little to no idling, but the energy required to accelerate to higher speeds is greater.
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What You'll Learn
Fuel efficiency is maximised when acceleration and braking are minimised
Fuel efficiency is a crucial aspect of vehicle performance, and it is well-known that maximising fuel efficiency offers significant benefits, both economically and environmentally. While the percentage of fuel energy used to accelerate a car varies with the vehicle, it is generally accepted that fuel efficiency is maximised when acceleration and braking are minimised. This is due to the fact that during acceleration, a car generally achieves better fuel efficiency as the RPM increases, reaching its peak near maximum torque. However, if a driver accelerates too quickly without paying attention to the road ahead, they may need to apply the brakes and then accelerate again, leading to reduced fuel efficiency.
To maximise fuel efficiency, drivers should aim to anticipate traffic conditions and drive in a way that minimises the need for acceleration and braking. This can be achieved through techniques such as coasting, which involves easing off the throttle early when approaching a red signal, allowing the vehicle to slow down and coast, potentially avoiding a complete stop if the light turns green in time. This technique prevents energy loss from braking and restarting the engine. While coasting, a driver can also shift to neutral to further reduce fuel consumption, although this practice is illegal in some jurisdictions and may require advanced driving skills to maintain safety.
Another strategy to maximise fuel efficiency is the pulse and glide (PnG) driving technique, which involves accelerating to a certain speed ("pulse" or "burn") and then coasting or gliding down to a lower speed before repeating the sequence. This method has been proven to save fuel, with some experiments showing up to a 20% fuel saving. Additionally, drivers can improve fuel efficiency by maintaining an appropriate distance from the vehicle ahead, reducing the need for sudden braking and acceleration.
Furthermore, advancements in vehicle technology play a significant role in enhancing fuel efficiency. Integrated starter/generator (ISG) systems, commonly found in hybrid vehicles, improve fuel economy by eliminating idling. These systems automatically turn off the engine when the vehicle comes to a stop and seamlessly restart it when the accelerator is pressed. Additionally, new tyre designs and materials can reduce rolling resistance, leading to increased fuel efficiency. However, these improvements must be carefully balanced against factors such as traction, durability, and noise.
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Energy-efficient driving techniques
- Keep your tires properly inflated. Under-inflated tires can increase fuel consumption by up to 4% and reduce the life of your tires.
- Remove unnecessary accessories and cargo from your vehicle. The more weight in your vehicle, the more fuel is needed to move it.
- Minimise the use of air conditioning. Even at high speeds, it is more fuel-efficient to open a window than to use air conditioning.
- Avoid roads with heavy traffic, as stop-and-go driving uses more fuel.
- Keep a comfortable distance from the vehicle in front of you. This will allow you to adapt your speed without using the brakes, which wastes forward momentum.
- Avoid idling. If you are stopped for more than 60 seconds, turn off your engine.
- Drive at a steady speed. Speeding up and slowing down frequently uses more fuel.
- Use cruise control when possible.
- When accelerating, take five seconds to accelerate to 20 kilometres per hour.
- Use the highest gear possible. Driving at high revs increases fuel consumption, so change up early when accelerating.
These techniques can help improve fuel efficiency and reduce carbon emissions, saving you money on fuel and improving road safety.
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Engine and driveline inefficiencies
In a conventional vehicle, only about 12%–30% of the energy from the fuel is used to move it forward, with the rest lost to engine and driveline inefficiencies or used to power accessories.
The drivetrain, or driveline, is the sum of components that deliver engine power to the wheels. It includes the clutch (or torque converter), gearbox, propeller shaft, differential, and drive shafts. The efficiency of the drivetrain impacts the overall efficiency of the vehicle, with higher drivetrain efficiency resulting in lower fuel consumption.
In gasoline-powered vehicles, most of the fuel's energy is lost in the engine as heat. Engine friction, pumping air into and out of the engine, and combustion inefficiency also contribute to energy losses. These losses can be reduced through advanced technologies such as variable valve timing and lift (VVT&L), turbocharging, direct fuel injection, and cylinder deactivation. Diesel engines are inherently more efficient than gasoline engines, with lower losses and up to one-third higher efficiency.
The transmission and other parts of the driveline also contribute to energy losses. Technologies such as automated manual transmissions (AMTs), double-clutch, lock-up transmissions, and continuously variable transmissions (CVTs) can reduce these losses. Additionally, electrical accessories such as seat and steering wheel warmers, lights, windshield wipers, navigation systems, and entertainment systems draw energy and reduce fuel economy.
The efficiency of the driveline can be improved by utilizing lockup devices in the higher gears, resulting in efficiency improvements of 5-10% over combined EPA cycles. Matching the road load power and velocity demands to the engine characteristics can also lead to significant gains in fuel economy.
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Rolling resistance
The motion of a car is resisted by several factors, one of which is rolling resistance. This is the force resisting the motion of a body with a rounded shape, such as a ball, tire, or wheel, as it rolls on a surface. It is caused by the deformation of the tire as it rolls on a flat surface, and the non-elastic effects of this movement. In other words, not all the energy needed for the tire to regain its shape after being deformed is recovered.
The materials used in the composition of the tire also influence rolling resistance. Tires with fillers and polymers can improve traction while reducing hysteresis. Replacing carbon black with silica-silane is one way to achieve this. The use of exotic materials, such as nano-clay, has also proven effective in reducing rolling resistance in high-performance rubber tires. Furthermore, the angle between the plane of the tire and the direction the car is traveling, known as the slip or scrub angle, impacts rolling resistance. A larger slip angle results in higher rolling resistance.
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Aerodynamic drag
To reduce aerodynamic drag, manufacturers work on the external components of a vehicle, such as the roof rack, mud flaps, rear spoiler, side mirrors, and radio antenna. They also employ design elements such as character lines on the exteriors of the car's body, which help air pass more smoothly over the body, reducing air resistance.
Additionally, smoother vehicle shapes can significantly reduce drag. Further reductions of 20%–30% are possible through improved design. However, it is important to balance these improvements with other factors such as traction, durability, and noise.
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Frequently asked questions
Only about 12%–30% of the energy from the fuel in a conventional vehicle is used to accelerate it and move it forward. The remaining energy is lost to engine and driveline inefficiencies or used to power accessories.
Acceleration has a direct impact on fuel efficiency. Higher acceleration can lead to quicker acceleration to a fixed target speed, resulting in higher average fuel economy. However, minimising acceleration and maximising coasting time can also improve fuel efficiency.
There are several techniques for improving fuel efficiency, including:
- Keeping tires properly inflated
- Maintaining your vehicle
- Avoiding idling
- Using regenerative braking (for hybrid/electric vehicles)
- Using advanced technologies like variable valve timing, turbocharging, and direct fuel injection
