Jean Billingsley | Course Facilitator
4th November, 2018
“Why can’t engineers answer a simple question?” is a typical remark I hear from someone who works with engineers. As an engineer myself, I would reply that engineers can answer a simple question – they just need enough information to do so.
Consider the question, “How many miles per gallon (mpg) does my car do?”. A showroom sales executive might give you an answer, such as “60 mpg”. However, this will probably be based on standards, such as the European Regulations, which define very specific tests for fuel consumption. If your underlying question is “how does the fuel economy of this car compare to other cars?”, then this is a useful figure.
However, these standard figures are not representative of real life and are rarely achieved – fuel consumption is usually much lower and depends on a number of factors. Fuel economy for urban driving is very different to driving on the motorway – the former is typically short distances at slow speeds. Driving styles and routes travelled affect not only average speed, but also the number of cycles of accelerating then decelerating and the amount of time spend idling. There are also other external factors that affect fuel economy, such as road and weather conditions. Hence, a realistic mpg is likely to be somewhat lower than the standard test figure.
If you really want to know exactly what you get from your car, then an engineer might suggest you measure it. You could note the mileage and how much fuel you have added to your tank. For a more accurate and representative measure, you could record this for several visits to the petrol station. You could then calculate the average mpg from the miles travelled divided by the gallons of fuel used. This would give you a fairly accurate measure of your car’s mpg.
So back to the original question, “How many miles per gallon (mpg) does my car do?”. If you are asking an engineer, they might come back to you with more questions and requests for information. If the question is not precise, they might need to understand the underlying problem, the specific conditions and how accurate you need the answer to be.
If you are working with engineers, you need to communicate clearly with them, and give them the information they need to know. Engineers are great problem solvers, but this does require a precise problem definition. Similarly, engineers are great designers, but this does require a detailed design specification.
Problem-solving and designing are two examples of situations where engineers use a logical, methodical process. The first steps in analysing a problem include understanding the problem and what information is available. A simplified, general mathematical approach to finding an unknown quantity, which engineers might use is:
1. State the problem clearly.
2. List the information that you have.
3. Identify the relevant physical laws or equations.
4. Calculate the answer.
5. Check the problem is addressed.
We could apply this to the question of finding your mpg:
Step 1 - The first step is to state the problem clearly. Let us assume that you really want to know an accurate mpg that you are getting for your car and how you drive it (and not the ‘standard’ for a general comparison or an approximate value).
Step 2 - Gather the information. Keep a record of the amount of fuel you put in your car and the mileage on each occasion. Let us assume you record this for six consecutive visits to the petrol station, filling the tank on each occasion. Note the correct start and end points: total miles is mileage at sixth fuelling less the mileage at the first fuelling, and total fuel consumed is the sum of the first five times.
Step 3 - The equation for miles per gallon is straightforward: mpg = miles/gallons.
Step 4 – Calculate. If you purchased your fuel in litres, you might need to convert the units to gallons.
Step 5 – A simple check might be the order of magnitude (typically in the range 20 – 80 mpg) or that it is close to the standard test figure. Then we will have answered the question “How many miles per gallon (mpg) does my car do?”.
If you are a non-engineer interested in learning about mechanical engineering, check out our Mechanical Engineering for Non-Mechanical Engineers course here. This course provides non-mechanical engineers and other professionals with an introduction into the core subject areas of mechanical engineering.
About the Author
Jean Billingsley is the course facilitator for Mechnical Engineering for Non-Mechanical Engineers.
Jean Billingsley has over 25 years of experience in mechanical engineering and project management, including within senior management positions at Rolls-Royce and Alstom. She has been delivering engineering and project management training for multinational companies such as Shell, Total, Petrofac, E-On, Jaguar Land Rover and many others.
She is a Fellow of the Institution of Mechanical Engineers (IMechE), a Chartered Engineer and a Chartered Environmentalist. She has earned an honours degree in Mechanical Engineering, has an MBA from the Warwick Business School, and is a Chartered Manager and Fellow of the Chartered Management Institute. She is on the Council of the IMechE and sits on the Strategy Advisory Committee.
She has worked with Voluntary Service Overseas (VSO) in the Philippines, in partnership with Renewable Energy and Environmental NGOs, on the technical management and program administration of micro hydro power and renewable energy systems. Recently, she has become a Director of the Environmental Sustainability Rotarian Action Group. She is a reviewer for the Management Book of the Year, a judge for multiple technical awards, and author of the book “So What? Effective Writing for Engineers”.