Dear Pablo, I am in the market for a new car. I see that there are some very fuel efficient diesel models available. Should I consider diesel over hybrid?
News of Volkswagen’s 71 mile per gallon Polo Bluemotion sure got some attention. Several other diesel models have recently also made the question of diesel vs. hybrid more relevant. While hybrid technology is relatively new, and relies on heavy and expensive batteries, diesel technology has been around since its invention by Rudolf Diesel in 1893. With new hybrid and diesel models coming out constantly, now is a good time to examine the relative benefits of each.
Diesel: Reputation vs. RealityDiesel engines work differently than gasoline engines. Rather than relying on spark-ignition to ignite highly flammable gasoline, the diesel engine uses compression-ignition, basically compressing a mixture of air and diesel fuel until its combustion is induced with the help of a glow plug. Diesel engines have a very high compression ratio and therefore have the highest thermal efficiency of any internal combustion engine.
You would be forgiven for associating diesel engines with plumes of black smoke pouring out of mufflers. Recent legislative advances have drastically improved the fuel standards for Ultra Low Sulfur Diesel (ULSD), cutting down exhaust emissions (non-greenhouse gas emissions like SOx) by 90%. New pollution control technology also helps reduce emissions from the tailpipe. The engine that was once associated with poor air quality and smog is now approaching the emissions levels of gasoline engines. Diesel fuel does have a higher carbon content that gasoline and results in 22.2 pounds of CO2 per gallon compared to 19.4 for gasoline.
Hybrid: Reputation vs. RealityHybrid vehicles have gained an aura of “greenness” and the popularity of the Toyota Prius has been fueled by conspicuous consumption. Car commercials promoting hybrids have tried to give the impression that driving a hybrid is equal to doing something good for the planet, when in reality it just amounts to doing something less bad. Driving an SUV is still consuming a lot more gasoline and emitting a lot more greenhouse gasses than is really needed to transport its solitary occupant from point A to Point B, even if that SUV is a hybrid. So let’s be clear, hybrid cars are a step in the right direction but they are not the solution to the challenge posed by climate change.
What about Electric?Plug in hybrid electric vehicles (2012 Prius Plug-In), range-extended electric vehicles (Chevrolet Volt), and pure electric vehicles (Nissan Leaf, Tesla Roadster) have only recently become serious participants in the market and it is estimated that 10% of vehicles on the road by 2020 will be powered by batteries instead of petroleum fuels. Obvious limitations in range will keep purely electric cars in the realm of commuting and getting around town, but that is what most cars are used for anyway. Range-extended electric vehicles use a backup motor to charge the batteries when they get low, allowing a much longer range and making them feasible for road trips longer commutes as well. The problem with all these vehicles is that they rely on lithium batteries, which are quite expensive, making the cars themselves fairly pricey.
What's The Verdict? Is Diesel Better Than Hybrid?When looking at fuel economy and base price of several currently-available diesel and hybrid models we find that hybrid vehicles cost more than their gasoline counterparts but don't always get impressive fuel economy, which is especially evident with hybrid SUVs. We also find that diesel vehicles can be almost as efficient as comparably-sized hybrids, mainly because of their efficient engines and lighter weight. An additional consideration is that newer diesel vehicles can consume biodiesel without modifications, making their carbon dioxide emissions "carbon neutral."
The ideal choice in vehicle is obviously based on many personal variables but the advantage of modern diesel cars is quite clear. Their ability to match the fuel economy of sometimes more expensive hybrid vehicles and their comparatively lower curb weigh (due to the absence of batteries) make diesel vehicles a great alternative to hybrids. Of course no fossil fuel-powered car, no matter how efficient, can be a solution to climate change. Eventually cars powered by electricity from renewable sources or by fuel cells using sustainably generated hydrogen will provide the next step forward, especially if they are manufactured using renewable energy as well. Ultimately though, trip avoidance (telecommuting, online shopping, etc.) as well as user-friendly and accessible mass transit are the true solutions to sustainable mobility.
Pablo Päster is a weekly columnist for TreeHugger.com and Principal Environmental Consultant at Hara Software. Send your questions to Pablo(at)TreeHugger.com or submit via this form and connect to his RSS feed.