What Car? E10 fuel tests

* New E10 eco-fuel is on its way to UK forecourts * E10 is a blend of petrol and up to 10% bioethanol * What Car? tests indicate emissions and fuel economy penalties...

Author Avatar
Emma Butcher
06 February 2014

In an effort to see what effect using E10 petrol actually has on cars using it, we conducted tests on four different cars. 

We compared E10 with E0 ‘pure’ petrol so that they would provide clear evidence that adding ethanol to petrol can reduce fuel economy. It’s important to stress that drivers using the current E5 grade are unlikely to see overheads increase by quite as much as the sums quoted here. However, it’s clear that increasing ethanol content in petrol can drastically increase fuel consumption.

We put E10 to the test in four cars, each with different engine characteristics. Our line-up included a three-cylinder turbo 0.9-litre Dacia Sandero, a 1.4-litre four-cylinder naturally aspirated Hyundai i30, a 1.6-litre four-cylinder turbo Mini Paceman, and a 1.8-litre Toyota Prius Plus hybrid.

We then put them through rigorous emissions tests using the E0 and E10 petrol to gain a clear picture of the effects of ethanol. E10 proved less efficient than E0 in all our tests. The average fall was -8.4%, equating to more than two extra tanks of petrol every year. Assuming both fuels were priced the same, it would represent an extra cost of £170. Apply this to all compatible cars and you’re looking at a national annual fuel bill hike of £3.4 billion. The effect would be smaller, but still significant, when compared with E5.

E10 test results
In our tests, the 89bhp Dacia Sandero struggled most, returning an 11.5% drop in mpg. That’s an extra cost of around £202 every 12,000 miles. The 99bhp Hyundai i30 was nearly as bad, managing 9.8% fewer miles on E10 than E0, an extra £16 a month.

The 134bhp Toyota Prius+ with its bigger hybrid engine fared better, using 6.4% more E10 than E0, while the 181bhp Mini Paceman was least affected by the ethanol; its fuel consumption increased by 5.9%.

E10 test conclusions
This would seem to suggest that more powerful cars cope better with a higher ethanol content, leaving small-engine models – often bought by drivers on tighter budgets – worst affected. It could explain why our results differ from the US Environmental Protection Agency’s estimate; many US cars still use big V6 and V8 engines.

There are other hypotheses, such as sensors reacting to the ethanol’s higher oxygen content, the ECU misdiagnosing lean engine running and injecting more fuel to compensate.

We believe every such theory must be investigated before E10-grade petrol is rolled out to the market, because even if you halve the shortfall in economy produced in our tests to reflect the leap from the E5 we use today to E10, it’s still significant.

It isn’t only fuel economy that’s affected. Ethanol might produce less CO2 compared with petrol in a lab, but due to the much higher consumption rate of E10 in our tests, tailpipe emissions were higher in every case. The Dacia pumped out an extra 11g/km of CO2 when running on E10, while even the less sensitive Mini’s emissions increased by 2g/km.

That said, E10 does have advantages in that bioethanol is renewable and the crops used to produce it absorb CO2 while growing, which should help offset some of those higher tailpipe emissions. That’s the view of the Low Carbon Vehicle Partnership (Low CVP). MD Andy Eastlake said: ‘The point of biofuels is not necessarily to reduce emitted CO2 but the fact that carbon is absorbed in growing the fuel means the net impact to the atmosphere is reduced versus extracting oil from the ground.’

According to Eastlake, the effect of the carbon absorbed by biofuel crops is a 3-4% reduction in the ‘well to wheel’ impact of the vehicle. This estimate does not, of course, take into account the increased level of CO2 the cars in our tests emitted on E10.