Automotive: Hybrids vs. Clean Diesels

With gasoline prices above $3/gal in the U.S., automotive buyers looking for the best fuel economy have the options of a hybrid or diesel powertrain instead of the conventional internal combustion engine. At this point, manufacturers offer far more hybrid than diesel-equipped models in the U.S., but in Europe, more than 50 percent of the vehicles have diesel engines. Across the Atlantic, diesels have sound footing but across the Pacific, in Japan, the hybrid is moving fast.

By the Numbers

There are a lot of perception versus reality issues for both hybrids and diesels. Diesels have a reputation as a noisy, smelly, black soot-producing powertrain. Their high torque makes them a natural for heavy-duty trucks and buses — typically slow moving vehicles because of the heavy loads they haul. However, automakers have attacked the diesel image with technology and solved age-old problems. Today's diesels start quickly, do not smell, generate 90 percent less noise, get excellent fuel economy and do not produce black soot thanks to electronic control and other advances.

As a newer technology, hybrids have a somewhat different image. Hybrids are cleaner based on the electrical/electronic portion of the drivetrain. Their newness has created other concerns including battery life, increased complexity (for increased repairs), cost versus payback, trade-in value, EPA versus actual mileage and more.

By 2012, a global hybrid market of 2.2 million units per year is expected, according to Japanese technology consulting firm Nomura Research Institute. Sales in 2007 were forecasted at 619,000 units. The U.S. is projected to have the leading hybrid volume of 1.68 million units with Japan second at 460,000 units. Europe is expected to have sales of only 52,000 units. In contrast, more than 15 million diesels could be sold globally in 2010 with more than 10 million in Europe. Diesels were less than 4 percent of U.S. sales in 2006. Global vehicle sales in 2007 should be about 70 million vehicles growing to about 85 million units in 2012.

The driving force behind both the diesel and the hybrid is improved fuel economy. No matter where the car company came from, a strength in hybrids or strength in diesels, they are all looking at the other technology and have advanced research efforts in both arenas. “I think that all the automakers are spreading their bets on all technologies,” says Kregg Wiggins, vice-president North America Powertrain, Continental, a maker of both diesel and hybrid components and systems. “There is no panacea from a fuel economy, power and overall standpoint.”

Smaller, reduced displacement engines are key to better fuel economy in both diesel and gasoline engines. Turbocharging and direct injection compensate for the reduced displacement. “The direct injection allows increased torque for smaller engines and the turbocharger helps develop the power,” says Wiggins.

The newest technology for driving the injectors uses piezoelectric technology. “With piezo technology, the main advantage is very high speed,” says Wiggins. “You can get several pre- and post-injections, if you like, in the same combustion cycle to allow different types of smoothing — smoothing of emissions, smoothing of power.”

Advances like improved direct injection are among the reasons the diesel engine is so popular in Europe but much work remains. “There are some challenges that face the diesel,” says Charlie Freese, executive director of diesel engine engineering for GM Powertrain. “Basically, the challenges are centered around emissions and cost.”

Continental's Wiggins agrees. “From the diesel side it comes down to cost but most would say that it is technically feasible,” he says. For hybrids, the main problem is the batteries. Wiggins admits there is a lot of effort and a lot of people who have very optimistic viewpoints there will be some breakthroughs in this area.

In the near term, GM's powertrain roadmap counts on advances in conventional engine technologies: internal combustion engine, gasoline or diesel and then transmissions. “That develops with a lot of new technologies that continue to push the state of the art and move toward ever more efficient, cleaner powertrains,” says GM's Freese.

For example, GM's new 4.5-l turbocharged diesel improves engine efficiency by 25 percent. The four-valve V-8 takes the same space as the company's small block V-8 gasoline engine. “When we set out with a clean sheet to design an all new engine, we wanted to make sure that it was somewhat, from a packaging standpoint, interchangeable with comparable gasoline engines that are already in our vehicles so that we minimize the amount of tear up to put the powertrain in place,” says Freese.

The diesel also reduces CO2 by 13 percent over a gasoline and has at least 90 percent reduction in particulates and NOx compared to today's diesel-equipped vehicles thanks to the engine's design, control and aftertreatment systems. The engine uses a selective catalytic reduction (SCR) NOx aftertreatment system with a diesel particulate filter to help achieve the Tier 2 Bin 5 and LEV 2 emissions standards in the U.S.

To address the increased cost, GM has an economic model linked to the price of fuel and the operating cost of the powertrain to show the difference between the U.S. and Europe markets and the added purchase price of a diesel versus a gasoline engine. On a price per gallon basis, $1/gal lower diesel prices in Europe are not unusual based on fuel taxation policies. With a cost premium of $2,000 over a gasoline engine, a diesel can pay back the added initial cost in 25,000 miles on a 15-mpg gasoline engine and with a 35-mpg unit it is 40,000 miles — well within the lifetime of typical buyer ownership, according to Freese.

With a 35-mpg vehicle in the U.S. and comparable prices for gasoline and diesel fuel, the same added purchase cost payback is about 200,000 miles. For a 15-mpg vehicle, the payback can be around 70,000 miles which can start to justify the added expense of the diesel. In many locations in the U.S., diesel fuel can cost about $0.50 more than gasoline. However, the comparisons are under light-loaded normal driving. If the vehicle is used for work or hauling, the fuel efficiency for the diesel is much greater. Another North American cost-related problem is more difficult to solve. “In North America, the powertrains required to move bigger vehicles are larger and the added cost of emissions compliance tends to push you further than the $2,000,” says Freese.

Beyond the Numbers

The hybrid's reputation is built on its green image even though it uses a gasoline engine and, unfortunately, the diesel has emissions challenges. GM's Freese points out the gasoline and diesel engines are reciprocals of each other. The gasoline has an efficiency challenge but the emissions solution is essentially in hand. The diesel is just the opposite. “We have the fuel efficiency inherently and our challenge is to push toward lower and lower emissions cost effectively,” he says.

Fun to drive is a vehicle aspect that is difficult to quantify. “From a consumer standpoint, I think the HEV and the diesel both give you that torque off the line feeling,” says Continental's Wiggins.

In perhaps one of the first head-to-head comparisons, Consumer Reports, November 2007 issue, put both the diesel and a hybrid to the test. Testing of the Lexus GS 450h and the Mercedes-Benz E320 BlueTec diesel, gave the diesel the initial price and fuel economy edge and the hybrid the acceleration advantage. Several other vehicle differences, independent of the powertrain, leave the final decision to the consumer.

However, the reception to hybrids that took advantage of the electric motor to provide performance at the expense of fuel economy has been less than projected and vehicle sales have been well below expectations. In addition to vehicles that have already been introduced, many automakers have used hybrid technology for concept vehicles to provide increased horsepower with a reduced emissions impact. We'll have to see how that works out.

One final issue to consider is resale value. Diesel engines are known to have a long life, as much as 50 percent or more, and that typically translates into higher resale value than vehicles with gasoline engines. Hybrids have not fared well compared to the cheaper gasoline-only version when it comes to resale value. Recent data shows the Honda Civic Hybrid costs more than its V-6 gasoline counterpart but owners will recoup less of that cost (known as a lower residual) when the vehicle is sold, according to the 2008 Automotive Lease Guide. The same is true for the Toyota Camry, Nissan Altima and Mercury Mariner hybrids versus gas-powered vehicles.

One thing is true, with more and more vehicle choices, consumers are already starting to shape the winners and losers. Technology advances and breakthroughs in either the hybrid or diesel area could tip the scale.