Posts from November, 2009

Rob’s Hypermiling How To Guide 02 – Fuel

This is Chapter 2 in a series of posts on Hypermiling. In Chapter 1 I introduced the concept of energy flow analysis as a systematic way of investigating driving techniques and vehicle modifications for improved mileage. I am  considering a vehicle as a closed system with energy input in the form of fuel and several energy outputs as shown in the following energy flow diagram:

Vehicle Energy Flow

In this post I will look at the only energy input: fuel.

Work, Energy, and Power

Before looking at fuel, it’s worth defining a few terms that I will be using throughout this series of posts. “Work”, “energy”, and “power” are terms used frequently and sometimes interchangeably by the general populace, but in an engineering context, they have specific meanings that must be understood. For a greater understanding than I provide here, follow the links to Wikipedia articles.

Work is a force applied over some distance. The amount of work is equal to the force multiplied by the distance. If force is measured in Newtons (N) and distance is measured in metres, then multiplying force by distance will give work in Joules (J). The Joule is a measure of energy. Work is a specific kind of energy. It can be thought of as energy used to move something.

Energy is a measure of the capacity to do work. It is also measured in Joules. Energy can take many forms (e.g., chemical, light, heat, work). Energy can be converted from one form to another through various means. In an engine, for example, the chemical energy of a fuel is converted to heat through the process of combustion and the heat is used to expand a gas against a piston, converting some of the heat to work.

Power is a “rate” of energy flow. Power is measured in Watts (W). It can be expressed as energy per unit time. 1 Watt is equivalent to 1 Joule per second. Since work is a form of energy equal to force times distance, and power is equal to energy divided by time, it follows that power is equal to force times distance divided by time. In other words, power is equal to force times velocity.

Just as power can be expressed as energy per unit time (e.g., 1W = 1J/s), energy may be expressed as power multiplied by time (e.g., 1Ws = 1J, 1Wh = 3600J or 3.6kJ, 1kWh = 3.6MJ) . You may be more familiar with energy expressed in kWh as this is a common unit of measurement used on utility bills.

I cannot emphasize enough how important it is to understand these terms and the formulas relating them. Without such an understanding, hypermiling is all just trial and error.

Energy Density of Fuels

A certain volume of fuel contains a certain amount of chemical energy that can be released by combustion. Energy density is a measure of the chemical energy per unit volume or per unit mass of fuel. Energy is specified in kWh (recall 1kWh = 3.6MJ), volume is specified in litres, and mass is specified in kg. Thus energy density of fuels is commonly specified in kWh/litre or kWh/kg.

The table below shows energy densities for some fuels you may be familiar with:

Fuel Density
[kg/litre]
Energy Density
[kWh/litre]
Energy Density
[kWh/kg]
Diesel 0.850 10.9 12.8
Gasoline 0.702 9 12.8
Propane 0.510 6.6 12.9
Ethanol 0.789 6.1 7.7

I drive a gasoline vehicle, so for every litre of fuel consumed, 9kWh of energy is input to the vehicle and the law of conservation of energy requires that all energy losses in the vehicle energy flow chart above must total 9kWh. Hopefully it’s clear that the way those 9kWh of energy are divided between the various energy losses will have a significant effect on vehicle mileage. Of specific interest is the fraction of energy “spent” on overcoming rolling resistance and drag since those are the only “necessary losses” to move the vehicle.

Aside: Whenever I encounter energy specifications like this, I like to do a quick cost comparison. For example, I know from my utility bills that I pay about $0.07 per kWh for electricity. I pay about $1.00 per litre for gasoline. Since gasoline contains 9kWh per litre, I effectively pay $1.00/9 = $0.11 per kWh for gasoline. This is one among many of the reasons why I don’t burn gasoline to heat my home and why I’m considering building an electric vehicle.

Although it’s conceivable that the energy density of a fuel may be manipulated by additives, this is generally not attempted by hypermilers and would be a poor place to start for the beginner. Also note that energy density is not related to octane level.

Unfortunately it appears that our first stop on the energy flow diagram hasn’t yielded any techniques or modifications a driver can use to improve their mileage. However, the important thing to take away from this post is that the energy density of a fuel is fixed and that for gasoline specifically, it is 9kWh/litre or 12.8kWh/kg. I’ll be coming back to those numbers again in later posts to convert calculated energy losses back to litres of fuel consumed, which is what hypermilers are really interested in.

Stay tuned for Chapter 3 – Engine Losses which I promise will be more exciting since there ARE a lot of driving techniques and vehicle modifications you can use to improve engine efficiency.