How Diesel-fueled Hybrids Work

Image Gallery: Diesel Engines Diesel-powered vehicles have a dirty reputation in the United States; however, cleaner diesel engines are on the way. See pictures of diesel engines.
Image Gallery: Diesel Engines Diesel-powered vehicles have a dirty reputation in the United States; however, cleaner diesel engines are on the way. See pictures of diesel engines.
Joe Raedle/Getty Images

The rusty old diesel Mercedes-Benz sedan slowly creeps down the road. Its rattling engine can be heard from a mile away as the driver has to downshift to climb a hill while clouds of black smoke spew from its tailpipe.

This is what many Americans imagine when they think of a car with a diesel engine: Noisy, dirty, unreliable and often underpowered. Both diesel engines and the fuel that powers them are more expensive in the United States than in Europe, where about 50 percent of all cars are diesels. Also, emissions regulations in some states mean that diesel cars cannot even be sold there.


Modern diesel engines are moving away from the stereotype of being noisy and polluting, and new diesel technologies from automakers like BMW, Mercedes-Benz, and Volkswagen are producing cleaner diesel engines which can be sold in all 50 states.

The new diesels are cleaner, more powerful, and more reliable than ever before. In fact, some diesel engines offer incredible gas mileage -- the European 2008 BMW 120d Coupe offers an estimated 55 mpg (23.4 km/l) [source: BMW].

So what would happen if a carmaker built a diesel hybrid? How would it perform, what mileage could it achieve, and what impact would it have on the environment? In this article, we'll look at a few different takes on the diesel hybrid -- you may be surprised at how well they perform and save fuel, too.

Diesel Technology

The 2007 Chevrolet Volt concept is an example of a series hybrid -- but not a diesel-powered hybrid.
The 2007 Chevrolet Volt concept is an example of a series hybrid -- but not a diesel-powered hybrid.

A diesel engine and a gasoline engine are both are internal combustion engines. They both burn fuel to drive pistons, and they both have the same end result -- they provide the power that eventually turns the vehicle's wheels.

However, diesel engines are typically more efficient than gasoline engines, and there are a few mechanical differences between the two. First, diesel engines have no spark plugs -- air is compressed within the combustion chamber at such a high pressure that when diesel fuel is introduced, it ignites. Also, while gas engines use carburetors or port injection to deliver the fuel to the combustion chamber, today's diesel engines use what's called direct injection -- meaning the fuel is injected directly into the cylinder.


Compared to gasoline-powered vehicles, diesels are more fuel efficient, produce more torque and they tend to be more durable. On the other hand, they are often noisier, harder to start in cold weather and are more expensive than gasoline engines. For a much more complete understanding of diesel engines, you may want to read How Diesel Engines Work.

Hybrid vehicles, at least the ones that we're familiar with today, combine the attributes of a gasoline-powered vehicle with an electric-powered vehicle to achieve higher levels of fuel economy than a standalone internal combustion engine can. Typically, these cars feature gas-powered engines coupled with an electric motor that's powered by a large battery.

The two power sources found in a hybrid car come together in different ways. On a series hybrid, the gasoline engine is responsible only for charging the batteries; it is not directly responsible for turning the wheels of the vehicle. The engine turns a generator, the generator charges the batteries and the batteries power the vehicle's electric motor (or motors) to make the wheels turn. An example of this is the upcoming Chevrolet Volt.

On a parallel hybrid, the vehicle can be powered by either one of the two power sources. The power source the vehicle uses -- either the gas-powered engine or the electric-powered motor -- is dependent upon the current conditions, or rather what the driver is asking the vehicle to do. The Toyota Prius is an example of this kind of hybrid - it runs on electric power at lower speeds and turns on the gasoline engine for passing and higher speeds. Most hybrid cars on the road are parallel hybrids, and the cars discussed in this article follow this format as well.

If you're looking for a more detailed description of gasoline-powered hybrid technology, you may want to read How Hybrid Cars Work.

In the next section, we'll look at what it takes to build a diesel-powered hybrid, and examine why they're not on the roads yet.

Diesel Hybrid Development

The Volkswagen Jetta TDI is significantly more expensiv­e than the gasoline-powered Jetta.
The Volkswagen Jetta TDI is significantly more expensiv­e than the gasoline-powered Jetta.

Dave Buchko, a spokesman for BMW says "Hybrid technology can be applied to any sort of powertrain. From the perspective of a hybrid, it makes no difference."

In other words, technologically, a hybrid with a diesel engine wouldn't be all that different from one with a gasoline engine. Although diesel engines have higher compression ratios and generate more torque than gasoline engines, Buchko, whose company is developing a diesel hybrid, the BMW X5 SUV concept, said there aren't any challenges with building such a car.


However, cost may be a discouraging factor. Both hybrid cars and diesel cars are more expensive than the same vehicles with gas engines. A 2008 Honda Civic Hybrid costs about $7,000 over the base sedan, and a VW Jetta TDI costs about $5,000 more than the gasoline version.

"Putting the two systems together could make for a high-price vehicle," said Bruce Belzowski, an automotive industry expert at the University of Michigan.

"To get back the money you'd save on fuel economy, you'd have to keep it a very long time," stated Belzowski. He also claims that since many diesel engines get similar fuel economy to some hybrids -- even better in some cases -- that a clean diesel makes a hybrid unnecessary. Belzowski added, "If you got a diesel with a particulate filter, why would you need a hybrid?"

Automakers haven't looked heavily into diesel hybrids for a number of reasons. For one, according to BMW's Buchko, the CARB standards made it tough to sell diesel in cars in big markets like California and New York. Also, diesel fuel has always been cheap in Europe, making a hybrid diesel not as necessary. Cost is also a factor.

"Until very recently, [America's] gasoline prices were fairly low," Buchko said. "In Europe, they were historically much higher. People migrate to the cheapest fuel, and in Europe that's diesel."

He added that two-thirds of all BMW sales in Europe are diesels. But as fuel prices skyrocket everywhere, a hybrid diesel may be an attractive option for buyers soon, no matter where they live.

In the next section, we'll show you how the high torque of electric engines and diesel engines could combine to make for an amazing driving experience.

Driving a Diesel Hybrid

BMW X5 Vision Diesel Hybrid concept
BMW X5 Vision Diesel Hybrid concept

A diesel already achieves fuel economy superior to a gasoline engine. So what kind of mileage could an electric-assisted diesel achieve?

The Golf TDI Hybrid concept car, unveiled at the Geneva Motor Show this year, gets 69 mpg (29.3 km/l) in the European fuel economy test cycle, according to Volkswagen. Compare that to the Prius' 54 mpg (23 km/l) in the same test, and it's easy to see that adding a hybrid component can significantly boost mileage.


While they do tend to provide good fuel economy, until recently, diesels weren't known to be the cleanest method of getting around.

The hybrid Golf concept emits 90 g/km (0.32 lb/mi) of carbon dioxide - much less than the 104 g/km (0.37 lb/mi) emitted by the Toyota Prius and 116 g/km (0.41 lb/mi) emitted by the Honda Civic Hybrid. That's also far less than the Golf BlueMotion diesel -- another vehicle that uses AdBlue urea injection to reduce emissions -- which puts out about 119 g/km (0.42 lb/mi) of carbon dioxide.

Both diesel engines and electric motors have reputations for generating enormous amounts of torque. Together, they can easily move a big SUV like the BMW X5. The German company unveiled a twin-turbocharged diesel hybrid concept version of the X5 at the Geneva Motor Show. The 2.0-liter four-cylinder diesel engine has 220 horsepower and 295 lb-ft of torque overall. That's more torque than the 3.0-liter six-cylinder gasoline engine that powers the base X5.

"It's not huge, but it's enough to motivate the X5 fairly well," said BMW's Buchko. "With this concept, we're showing what's possible."

In the next section, we'll take a look at the pros and cons of diesel hybrid buses -- in fact, depending on where you live, you may have already taken a ride in one.

Diesel Hybrid Mass Transit

The Connected Bus, a new bus with several high-tech features, including a diesel hybrid system, is currently in service in San Francisco, Calif.
The Connected Bus, a new bus with several high-tech features, including a diesel hybrid system, is currently in service in San Francisco, Calif.
Justin Sullivan/Getty Images

If you live in a city like New York, Chicago, or San Francisco Calif., and you use public transportation, chances are good that you've already been a passenger on a diesel hybrid bus. Nearly all buses and heavy-duty large trucks use diesel engines because of the massive torque available -- torque helps the vehicle pull huge amounts of weight. Just like cars, they use either parallel or series hybrid systems to assist the diesel engines in these buses.

San Francisco's hybrid bus system, first unveiled in April 2007, uses a series hybrid system where the engine turns a generator that produces electricity to power electric drive motors that propel the bus. The wheels aren't turned by diesel power directly, but rather electricity.


Seattle Wash. King County Metro Transit buses run on a parallel hybrid system - this means that at low speed the bus is mostly electrically driven, while at medium speeds it operates on a blend of diesel and electric power and at high speeds it runs on mostly diesel power.

The benefit offered by buses powered with diesel hybrid technology is that they produce significantly lower emissions and increased fuel efficiency. One of the downsides is the cost -- they can be several hundred thousand dollars more expensive than conventional buses. In Seattle, according to King County's Web site, fuel economy in the hybrid-powered buses is more than 4 mpg (1.7 km/l). That's pretty good when compared to the 2.6 mpg (1.1 km/l) the conventional diesel-powered buses are currently getting.

Some dispute whether hybrid buses are truly worth the extra cost. In Seattle, it has been reported that the buses have never achieved the fuel economy they were supposed to -- and in some cases, they actually achieved lower fuel economy numbers than standard diesel buses. Like a diesel hybrid car, it's also difficult to argue whether the added initial cost of the buses is made up for in fuel savings.

In the next section, we'll take a look at some of the diesel hybrid concept cars that have made their way into the auto show circuit. Will one end up in your garage?


Diesel Hybrid Concepts

The Volkswagen Golf TDI hybrid concept.
The Volkswagen Golf TDI hybrid concept.

The Golf TDI hybrid concept packs a lot of fuel economy into a small package. It uses a three cylinder common rail diesel engine with a displacement of only 1.2 liters. It produces 74 horsepower and 132 lb-ft (178.76 Nm) of torque. In addition, the Golf TDI also has an electric motor that operates either with the diesel engine or separately. The electric motor is able to produce 27 horsepower and 103 lb-ft (139.49 Nm) of torque.

According to Volkswagen, the electric motor provides enough power to get the vehicle moving from a standing start. Once the vehicle is in motion, the diesel engine will only engage at higher speeds, or if extra acceleration is required. In order to further improve fuel economy, the Golf TDI also rides lower than the standard Golf on a revised suspension and makes use of a unique front bumper to reduce drag.


However, reports indicate that while Volkswagen is planning to produce a hybrid, it won't likely be this one. Citing the high cost of building a diesel hybrid as a factor, the company is anticipated to be going with a turbocharged and supercharged gasoline hybrid instead.

When it comes to high-end luxury sedans, you can't do much better than a Mercedes-Benz S-Class. The top-of-the-line four-door Mercedes offers massive amounts of performance, comfort, gadgetry, and style -- just about everything except fuel economy. The 2009 Mercedes S600 model only achieves 11 mpg (4.7 km/l) in the city and 17 mpg (7.2 km/l) on the highway [source:].

But at the Frankfurt Motor Show in 2007, the company showed off a concept S-Class diesel hybrid that can deliver more than 40 mpg (17 km/l). The Mercedes-Benz S300 BlueTEC Hybrid concept has a 2.2-liter diesel engine that drives the wheels most of the time, along with an electric motor to assist with acceleration. Overall, it produces about 224 horsepower and a whopping 413 lb-ft (559.32 Nm) of torque -- numbers that are comparable to a powerful V8 engine.

The S300 BlueTEC Hybrid concept also uses Mercedes' version of the AdBlue urea injection system, which allows the vehicle to meet the strictest of emissions standards. This is another potential 50-state diesel-powered car.

The 2006 Citroën C-Métisse concept car on display at the Paris Motor Show.
The 2006 Citroën C-Métisse concept car on display at the Paris Motor Show.

One of the more unique diesel hybrid concepts ever seen comes from a company known for innovation. French automaker Citroën showed off its C-Métisse concept car at the Paris Motor Show in 2006. The fire-engine red concept car was a low-slung four door coupe with swooping lines and huge wheel arches.

Better yet, the C-Métisse was a diesel hybrid with a very unusual setup -- a 2.7-liter diesel V6 in the front of the car drives the front wheels, while two electric motors drive the rear wheels. The V6 puts out about 208 horsepower and the two rear motors deliver about 295 lb-ft (399.51 Nm) of torque each, according to Citroën. That's enough to drive the vehicle from 0 to 62 mph (100 km/h) in about 6.2 seconds.

The car achieves about 45 mpg (19.1 km/l) with this setup. It can also run up to 20 mph (32.2 km/h) on electric power alone, where it operates in "Zero Emission Vehicle" mode. Citroën says the concept is very environmentally friendly, and is equipped with particulate control systems.

With their excellent performance, proven durability and great fuel economy, why wouldn't automakers produce diesel hybrid vehicles? Substantially higher initial cost may be the biggest factor, but as fuel prices climb, a diesel hybrid just may be in your future.

For more information about diesel engines, hybrid cars and other related topics, follow the links on the next page.

Lots More Information

Related HowStuffWorks Articles

More Great Links


  • A phone interview with Bruce Belzowski. Automotive Industry Expert, The University of Michigan.
  • A phone interview with Dave Buchko. Advanced Powertrain Spokesman, BMW.
  • Citroën. "C- Métisse: Diesel hybrid drivetrain."
  • Gartner, John. "Diesel Hybrids on the Fast Track." Wired News. March 21, 2005.
  • King County Dept. of Transportation. "Metro Transit rolls out the first new hybrid buses." May 27, 2004.
  • Nunez, Alex. "Frankfurt Preview: Mercedes-Benz S 300 BLUETEC Hybrid." Autoblog. Sept. 7, 2007.
  • Nunez, Alex. "Official Update: VW Golf TDI Hybrid." BioDieselNow. March 4, 2008. update-vw-golf-hybrid.aspx