# Reduce fuel consumption in your car

## Introduction

A car requires mechanical energy in order to work. This mechanical energy is provided by the combustion of fuel and O_{2} to give primarily Co_{2} and water vapor. The energy requirements of a car depend mainly on two factors:

-Its weight

-The speed of the vehicle

Indeed, according to one of the laws of physics: E=0,5*m.v^{2}. Which means:

Energy = 0.5*mass * velocity * velocity. Thus, a car that weighs 1500 kilos needs 1.5 times more energy than one that weighs 1000 kilos in order to move. And driving at 141 km/h (88 mph) requires: (141/100)^{2} = twice the amount of energy than driving at 100 km/h (62,5 mph). Driving for 30 minutes at 141 km/h (88 mph) means to drive 70 km (44 miles) consumes as much energy as driving at 100 km (62,5 miles) at 100 km/h (62,5 mph).

Therefore, if a car consumes 5 liters/hour at 100 km/h (62,5 mph), it will consume 10 liters/hour at 141km/h (8_ mph). That is, an increase in consumption from 5l/100 to 7,09l/100, which means from 47 to 33 MPG.

The faster we drive, the more we consume. The heavier the car, the more it consumes

## Optimize your fuel consumption

Yet, the cars of the '70s were lighter than those of today but consumed more. Why?

A car requires mechanical energy in order to move. During combustion, in addition to mechanical energy, fuel produces energy in the form of heat that is lost through the exhaust (Joule effect). The fuel consumption of a car varies according to its efficiency. Thanks to advances in science and technology, the cars of today are more efficient, hence the lower fuel consumption.

If on the whole, the faster we drive, the more we consume, then why do we consume less on the freeway, where we drive a lot faster, than in town?

The answer: The engine is calibrated so as to run at a certain speed according to the gear that you put the car in. There is little power in fifth gear, which explains why we cannot move off in this gear. However, once it has been engaged, driving in fifth is very economical, especially when traveling at around 60-65 mph. This is why there is higher fuel economy on the freeway than in urban areas. Another reason for this is that we accelerate and brake constantly in urban areas. Now, when we step on the brakes, energy is dissipated, and this energy must be replaced when we step back on the accelerator. Hence the overconsumption in town areas (up to 6 mpg when accelerating hard) with respect to the freeway where the speed is globally constant.

The amount of fuel consumption therefore depends on the engine efficiency, as well as the vehicle speed and the speed stability: the longer the speed remains constant, the less fuel is consumed. In order to consume less, one must drive at the optimum speed in each gear (in the case that your car is a manual car). For example, with a Renault 19, a French old car, for example, the optimum speeds are:

1st gear: 6 mph

2nd gear: 12 mph

3rd gear: 20-25 mph

4th gear: 30-37 mph

5th gear: 50-65 mph

This means that if we drive at above 65 mph or below 50 in fifth, more fuel will be consumed than if we drove at between 50 and 65 mph.

Listen to your motor to avoid ever overtaxing it.

## Summary

Let us summarize: The consumption of a vehicle depends mainly on:

The vehicle speed. The faster it goes, the greater the drag on the car.

Its efficiency. The Toyota Prius, for example, recovers energy dissipated during braking and optimizes the engine regime, hence its lower consumption

Vehicle weight.

Speed stability when driving

Thus, a car weighing 1200 kg (2645 lbs) and traveling at 56 mph (25 m/s) has an energy of: 0,5*1200*25*25 = 375 000 Joules, as opposed to 735 000 at 78 mph (35m/s). Going from 56 mph to 78 therefore consumes an additional 360 000 Joules, i.e. roughly 10 mL of fuel (25 mL if we take into account an efficiency of the motor of 40%). Drag issues aside, an acceleration and a deceleration result in an unnecessary loss of 25 mL of fuel. This energy wasted could have provided electricity for 2 hours of computer use...

If you have to vary your speed every 2 miles on a freeway due to heavy traffic, you will consume (600 miles/2)*25mL = 7.5 Liters (2 gallons) of fuel unnecessarily on a trip between Los Angeles and San Fransisco. In this case, opt for a constant speed of 90 instead of alternating back and forth between gears.

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