An illusion: hybrids' gain over 'frugal diesels”

The advantage of hybrid vehicles over frugal diesels can be an illusion, if the judging criteria are based solely on fuel economy, according to Emissions Analytics – a leading provider of real-world emissions measurement.

Emissions Analytics has tested over 30 hybrids in the UK and US, and analysed thousands of data points to understand how such vehicles really perform in the real world, and compared them to vehicles with conventional internal combustion engines (ICE) only.

The data set compares two standard hybrids versus eight diesel engines, evaluated as part from the real-world fuel economy testing Emissions Analytics conducts with What Car? in the UK. Each model used an engine ranging from 1.5 to 2.2-litres, generating up to 150bhp, employed two-wheel drive, with a hatchback, saloon or estate body style.

The results (Fig. 1) show that while hybrids can deliver good fuel economy in real world driving, they can be eclipsed by up to 10 mile/gal by some non-hybrid diesels. This is after having taken into account any net changes in battery charge levels - to ensure that the hybrids are not penalised over the test cycle. 

For certain driving patterns, however, hybrids still prove the better option. Using Emissions Analytics’ complete dataset of more than 500 vehicles in the UK, it can quantify how average miles per gallon (MPG) can fluctuate when dealing with congestion and aggressive or fast driving.

Further data (Fig. 2) shows that hybrids suffer much less than their ICE equivalents in congested urban driving: on average a 3 per cent penalty compared to 7 per cent. In contrast, by doubling the average rate of acceleration, the MPG falls by more for hybrids – especially diesel hybrids. 

Comparing motorway driving to town driving, all types of vehicle show better MPG over faster routes. However, the difference between hybrids and ICE vehicles is dramatic – typically because the downsized engines found in the hybrids are less suited to motorway cruising.

Lower urban fuel consumption is not the only attraction of hybrid cars, as many people are motivated by their green credentials – and hybrids can deliver a significant emissions reduction in urban areas. “For diesel cars, we have previously found that low speed, stop-start driving dramatically increases levels of NOx emissions,” says Nick Molden, chief executive officer of Emissions Analytics.

A recent report by the International Council on Clean Transportation, which analysed data from Emissions Analytics, showed that the latest Euro 6 diesel cars can exceed the NOx targets by an average of seven-fold.

This contrasts with gasoline-only vehicles, which generally meet the regulated NOx standards – even in real-world driving. Carbon monoxide emissions are generally higher for petrol-only vehicles, but still within the regulated values. As a result, petrol hybrids have the benefit over ICE diesels in their effect on air quality, enhanced by the fact that a proportion of urban driving will be on battery - with zero emissions.

Given the current debate about diesel car emissions in towns and cities, Emissions Analytics’ research confirms that hybrids, particularly petrol ones, could help to reduce many of the pollutants emitted in these conditions – most importantly NOx.

“Hybrids may deliver good but not best-in-class fuel economy, but they are typically the cleanest, and if you are a light-footed, congested-town driver, they are ideal,” explains Molden.

Fig. 1 – Hybrids versus diesels, ranked by fuel economy:

Make	Model	Engine Size	Derivative	Fuel	Transmission	True MPG
Honda	Civic	1.6	i-DTEC ES	Diesel	Manual	67.2
Skoda	Octavia	1.6	Greenline III TDi CR	Diesel	Manual	61.9
Peugeot	308	1.6	Allure BlueHDi	Diesel	Manual	60.8
Mazda	3	2.2	SE-L Nav Skyactiv-D	Diesel	Manual	59.4
Toyota	Auris	1.8	Touring Sports Icon VVT-I	Petrol hybrid	Automatic	58.7
Citroen	C4 Cactus	1.6	Flair e-HDI	Diesel	Automatic	57.8
Toyota	Yaris	1.5	Excel VVT-I	Petrol hybrid	Automatic	57.8
Peugeot	2008	1.5	Feline e-HDi	Diesel	Manual	57.7
Volkswagen	Golf	1.6	Bluemotion TDi	Diesel	Manual	56.8
Honda	CR-V	1.6	i-DTEC SR	Diesel	Manual	56.5

Fig. 2 – The effect of congestion and aggressive or fast driving on fuel economy

	Urban congestion penalty	Urban aggression penalty	Extra urban benefit
Diesel	-6.0%	-8.4%	18.4%
Diesel hybrid	-2.5%	-12.9%	1.1%
Petrol	-8.3%	-6.5%	27.4%
Petrol hybrid	-3.3%	-7.5%	8.5%