Thread: Displaying fuel economy / consumption in certain conditions

Hello everyone,

Within the ‘walls’ of Palmer Performance Engineering we are having a debate of sorts that could use some outside perspective. The discussion is about displaying fuel economy / consumption when it’s not possible to calculate a value because one or more of the input values are zero. This happens frequently with instantaneous fuel economy / consumption and can happen with average fuel economy / consumption in special circumstances. We are turning to you, our customers and potential customers, tell us what you want.

But before I go on I should define a few things.

Fuel economy – the US and English way of representing a fuel consumption rate, expressed as mile per gallon or mpg. It is a distance travelled divided by the fuel consumed to travel that distance.

Fuel consumption – the metric way of representing a fuel consumption rate, expressed as liters per 100 kilometers or l/100km. It is the volume of fuel needed to travel 100 kilometers.

Instantaneous (fuel economy / consumption) – a method of calculating fuel consumption rate based on current speed and current fuel flow rate. The formula for Instantaneous fuel economy is VSS / FUEL_RATE. For example, given a speed of 50 miles/hour and fuel flow rate of 2 gallons/hour we get 50/2 = 25 miles/gallon.

Average (fuel economy / consumption) – a method of calculating fuel consumption rate based on the total distance travelled within a time interval and the total fuel consumed within the same time interval. The formula for average fuel economy is DISTANCE_TRAVELLED / FUEL_CONSUMED. For example, if in a two hour trip you travel 100 miles and consume 4 gallons of fuel, the average fuel economy for your trip is 100 / 4 = 25 mpg.

Sorry for the review but I’ve found that many OBD-II software programs don’t make the distinction between instantaneous and average fuel consumption. Also, the average of all instantaneous readings is NOT equal to average fuel consumption.

From the formulas above it’s obvious that when certain values become zero the calculations become meaningless. Here are the various scenarios where an input value can become zero.

1. When the engine in a hybrid vehicle turns off and the vehicle runs solely from the battery, the fuel flow rate is zero.
   
   • Instantaneous fuel economy and fuel consumption can’t be calculated because there’s no fuel being consumed. The math for instantaneous fuel consumption works out to zero but it’s still a nonsense value.
   • Average fuel economy and fuel consumption can still be calculated, the distance travelled is increasing but the fuel consumed is static, and both values are non-zero which leads to a valid number. The average fuel economy would increase as expected and the average fuel consumption would decrease as expected.
   • If per chance a new interval for calculating average fuel economy / consumption is started, the fuel consumed would remain zero while the distance travelled increases. In this scenario, a value cannot be calculated even if the math for average fuel consumption works out to zero.
2. When a vehicle is stopped but the engine is still running and consuming fuel, the vehicle speed will be zero and the distance travelled will no longer be increasing. Also known as waiting for the light to turn green.
   
   • Instantaneous fuel economy and fuel consumption can’t be calculated because vehicle speed is zero. The math for instantaneous fuel economy works out to zero but it’s still a meaningless value.
   • Average fuel economy and fuel consumption is can still be calculated. The distance travelled is static while the fuel consumed is increasing. The average fuel economy would be decreasing as expected and the average fuel consumption value would be increasing as expected.
   • If per chance a new time interval for calculating fuel economy / consumption is started, the distance travelled would remain zero while the fuel consumed value would increase. In this scenario an average value cannot be calculated even if the math for fuel consumption works out to zero.
3. When the vehicle is ‘on’ but stopped and the engine is not running. Vehicle speed is zero, distance travelled is not increasing, fuel consumed is not increasing and fuel flow rate is zero.
   
   • Instantaneous fuel economy / consumption can’t be calculated.
   • Average fuel economy and fuel consumption is calculable but remains static.
   • If the time interval where average fuel economy / consumption is restarted then the average cannot be calculated.

Now that I’ve presented the various scenarios we can get to the crux of the matter… what to display when a value can’t be calculated?

The screenshot below shows where values for instantaneous fuel consumption (left) and average fuel consumption (right) are displayed.



Camp 1:

Display dashes (--.-) where the value would go when the value can’t be calculated or would result in a 0. NA was considered but was judged too strong and conveyed the message that the whole gauge was permanently not available instead of being just temporarily unavailable.

Camp 2:

Display 99.9 when a value can’t be calculated to represent infinity or a high number and display 00.0 for values where the math works out to 0.


Interestingly, the automakers seem to be divided on this issue as well. Ford and Volvo are in Camp 1 while GM and Toyota seem to be in Camp 2.

Someone even argued that GM and Toyota display 99.9 due to a limitation in the electronics and firmware of the built-in trip computers that can only display numbers. 99.9 becomes the representation of infinity or the highest number the firmware can display. But only GM and Toyota really know.

So… which camp are you in?

What does your vehicle do?

I look forward to your opinions and discussion points.

Pierre.