What’s the carbon saving from running vehicles on liquid petroleum gas (LPG)? Wilcon Energy, a company in Colorado, USA that converts taxi and bus fleets to run on propane / LPG needed accurate answers. They wanted to have engaging imagery to show potential clients the carbon saving compared to petrol or diesel.
It was a straightforward brief, but needed care in choosing exactly what to compare with what. And, as ever, we needed to look closely at who can provide accurate numbers that we could use to build the images.
We might have used the savings in CO2 emissions per gallon of fuel from LPG use, like the image above. But that comparison is unhelpful as LPG has lower energy density than diesel or petrol.
Much better to compare like with like, which suggests comparing the emissions per unit of energy. As we are dealing with vehicles, a distance travelled is a sensible substitute for energy used. So we created this image (below) to show the difference over 100 miles of driving .
Easy, then, to reframe to show the CO2 saving per 100 miles - as in this sketch.
Yes, but where do the numbers come from?
The summary makes it sound simple, but that’s only true if you can find definitive data on which to base the calculations. Not so easy in this case. In the past we have relied on the US Environmental Protection Agency (EPA) website. However there have been changes at the EPA - to understate the situation - and we found many links broken. So we went for UK Government figures instead. Click here for UK Government GHG Conversion Factors for Company Reporting
The difference between EPA values and UK Government values will be tiny, and certainly won’t affect the visuals.
The 100 mile calculation is a bit more complicated. As we said, comparing like with like means comparing the emissions per unit of energy, rather than per gallon of fuel. But we consider emissions per unit of energy too abstract for most audiences - hence our opting to show the energy needed to drive 100 miles. Here we started with a 2010 EPA study that showed average fuel consumption in gasoline cars was 21.5 miles per gallon. Combining this with EPA values for the energy content of gasoline gives a value of 157.3 kWh for 100 miles, so we converted that into CO2 emissions for each fuel, using the UK conversion factors. And then we convert again to non-metric units for a US audience. And that is what you see in the image above.
From UK Government GHG Conversion Factors for Company Reporting:
LPG: 0.21451 kg CO2e/kWh
Gasoline: 0.24048 kg CO2e/kWh
Diesel: 0.27588 kg CO2e/kWh
To find out more about Wilcon Energy see here