Diesels challenge hybrids for efficiency, low emissions
This trend was not only driven by environmental regulations of various governments but the fuel price itself. Also consumer behaviors have changed a lot and our society is undergoing a paradigm change with regards to our transportation concepts and mobility behavior.
These days more and more new car customers are looking at the datasheets and specifications for fuel consumption and emissions. The prestige and image of many cars are now largely driven by its benchmark fuel efficiency and emission values rather the pure horsepower backed driving performance and acceleration. So this changing consumer behavior and the strong government involvement via rules, regulations and incentives for environmentally friendly cars leads the automotive market towards a new generation of cars to satisfy this trend of “green transportation”. And that's the main reason why hybrid-electric Vehicles (HEVs) are showing up in every fleet and product line of the major car manufacturers.
What we often forget, especially here in the U.S., is that fuel efficiency and low emissions are not only achieved via revolutionary game-changing technologies like the emission-free electric powertrain of a hybrid vehicle. Looking at the fuel efficiency of currently available or shortly announced HEVs, you would be surprised that this fuel efficiency can be pretty closely matched by a modern diesel engine using state-of-the-art direct fuel injection systems and an electronically controlled and well optimized motor control unit. The big difference comes with the price "bump up" for the technology that the buyer pays.
A diesel engine typically adds only moderat cost compared to a comparable gasoline engine (in the range of approximately $1-2,000). The price adder of HEVs vs their traditional standard combustion engine version is normally at least 2-3 times as high. The reason for this is that hybrid-electric vehicles need to incorporate a complete electric powertrain plus all the energy management and storage systems, a high voltage battery, and plenty of electric systems like inverter, converter, and charger applications into the vehicle architecture.
Therefore the additional electronic content in a HEV is much higher than the little electronics in the motor control unit plus some diagnostic and monitoring sensors which are needed to build a very efficient direct fuel injection system for a modern diesel aggregate in a much more cost effective way.
Checking the numbers
Let's look at the table below, which shows the average highway fuel consumption in miles/gallon of HEVs vs diesel. There is not much difference between similar sized and powered diesel and HEV cars. The emission values of those cars are also in a comparable range. Only the pure city consumption of the HEVs seems to be superior since the HEV-engine typically shuts down (so called start-stop mode) every time the car is stopped in a traffic jam or at red lights. But adding relatively low cost start-stop functionality to a regular diesel engine (so called micro-hybrid) would push the diesel city mileage likely into the same range as the one advertised for gasoline-HEVs.
Looking at these numbers the diesel solutions could offer quite some threat to the penetration of HEVs. In countries in Europe where diesel is a well established engine type and the consumer acceptance is at least as high for gasoline powered aggregates we see already a very strong market penetration of diesel while HEVs seem to sell much harder compared to U.S. or Japan, for example.
In America the diesel engine still has the bad reputation of being loud, noisy, offering not much dynamic power, and having a very bad emission behavior. Plus based on some unsuccessful introduction attempts decades ago the diesel engine is also believed to be very unreliable and to easily break down compared to a gasoline motor. Unfortunately the countrywide diesel image in America seems to have stopped with this negative experience.
Not many people seem to realize or followed the tremendous success story of the diesel engine in Europe over the past decades. Today's turbocharged and direct fuel injected diesel engines don’t have to fear the comparison with any high end gasoline motor in terms of power, performance, quietness and comfort. Plus year of experience in Europe seem to have proven that a well engineered diesel engine is normally at least as reliable and rugged as its gasoline peers. Often diesel engines actually run many more miles over lifetime than a comparable gasoline motor.
So it is a pity that, in the U.S., people do not yet see the benefits and values of a diesel engine which could show their strengths, especially in the very common long-distance highway commutes.
Encouraged and supported by the recently released highly efficient “clean diesel” generations, like the BLUETEC concept from Mercedes, many European manufacturers are now trying again to enter the American market with their most recent diesel fleets. The efforts and improvements on emissions due to improved filter and exhaust technologies make the diesel comparably clean and environmentally friendly to gasoline cars—meeting the more stringent regulations of states like California. But still it will likely take quite some marketing efforts, promotion campaigns, and very credible advertisements to convince the population of diesel engine efficacy.
But if the U.S. market and car buyers indeed pick up the new trend and start valuing the benefits and the performance of modern European designed diesels, then suddenly North America could indeed become the next huge market for diesel cars. This on the other hand could lead to a major push back on the penetration ratio of HEVs in the market.
At least one big obstacle that can prevent the introduction and penetration of diesel engines in less developed/emerging countries should not be a problem for the U.S.—namely the availability of high-grade clean diesel fuel.
As will be explained later, modern diesel engines draw their performance out of a sophisticated motor control strategy that is based on high pressure fuel injectors. These highly precise masterpieces of electro-mechanical engineering can be contaminated or clogged by unsuitable fuel. So the availability of quality fuel and the corresponding infrastructure of fueling stations could become a major decision criterion whether a diesel engine may or may not be penetrating a country's market.
The HEV introduction and penetration in emerging countries like China or India is probably quite fast and not too much endangered by diesel since there are still some limits to a broad countrywide availability of clean fuel. These countries need environmentally friendly transportation means but the infrastructure and the availability of higher grade fuel seems to be problematic, especially when you look outside of the metropolitan areas. So countrywide improvements in the fuel supply chain would be needed, which now puts the diesel solution on par with electric vehicles—which themselves need major improvements in the electric supply infrastructure.
Coming back to the U.S., availability of clean diesel fuel is not an obstacle. This fact could give diesel an additional advantage vs the electrical infrastructure improvements that are required if the country wants to be ready for charging a major volume of plug-in hybrid or fully-electric vehicles. Therefore diesel seems to be a very logical and reasonable solution for our highway-driving nation. The diesel engine just needs to overcome the prejudice in the U.S. market.
For a power semiconductor manufacturer like International Rectifier with a focus on power management devices, this competition between HEV and diesel is a neutral situation—because the common denominator between HEVs and diesel cars is high voltage. The hybrid powertrain as well as the direct fuel injection system need very rugged and robust high voltage power switches and driver ICs. So developing technology, capabilities and power management solutions to address the high voltage range >>100V is beneficial to both HEV and diesel applications.
New activities of various car manufacturers target introducing this high pressure direct fuel injection concept into gasoline engines as well, simultaneously boosting fuel efficiency and reducing emissions. Power switches like so called IGBTs play a key role in such an application. They provide efficient power management and precise control over the piezo actuators in conjunction with optimized high voltage driver ICs, which need to be capable of driving the high voltage switches in a safe and reliable way.
Figures below show a simplified direct fuel injection system and the corresponding typical circuit diagram, respectively. High voltage is required to drive the piezo elements inside of the very precisely tailored fuel injectors. Those high tech injectors inject fuel often up to seven times per second in order to precisely tailor the fuel charge inside of the engine cylinders to an optimized gas phase. Distribution and gas density play an important role by optimizing the combustion process to the various power requirements in the driving cycle of the car.
An additional benefit
One important benefit of diesel engines has not yet been mentioned. It should be noted that the high fuel efficiency of HEVs is more or less a theoretical value that significantly depends on driving style. Unless the driver is adjusting his driving behavior to a very moderate use of the gas pedal in order to avoid the use of the inefficient gasoline engine, the HEV fuel performance can be much worse than stated and become closer to a less efficient gasoline combustion engine.
On the other hand a diesel engine is always very efficient by itself, what one could call a more “passive fuel efficiency concept” then the HEV philosophy, which relies on the driver maximizing the usage of the electric powertrain versus the gasoline motor. The performance data of the newest diesel generations suggest a much smaller dependency on the driver’s style. The motor control unit optimizes the fuel consumption to the current driving situation, which helps to achieve a high mileage independent of driver or driving conditions.
Diesel engines in well-developed countries can provide some threat to the penetration and growth of the hybrid-electric vehicle market. With diesel fuel supply infrastructure in place, attractive price, and excellent performance and efficiency, diesel cars can become a major game changer in countries where diesel is not yet very well established—like the U.S.
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