While the rotary engine has been abandoned by all vehicle manufacturers with the exception of Mazda, that company has not only continued to refine and optimize what it calls its RENESIS rotary engine—which is found beneath the hood of its RX-8 sports car—but Mazda engineers are quite literally taking the rotary engine to an all-new level by powering it with hydrogen. And according to Akihiro Kashiwagi, Hydrogen RE Development Program Manager, at Mazda Motor Corporation, the rotary combustion chamber, which proved to be so vexing for some auto manufacturers, actually has an ideal configuration for handling hydrogen fuel.
First of all, know that with comparatively minor modifications, a conventional internal combustion engine (ICE) can handle hydrogen. That's not an issue. So it's not that the rotary engine by its nature can do something that one with pistons can't.
But as Kashiwagi explained, the fact that hydrogen is highly combustible makes using it in a reciprocating ICE a bit tricky. That's because as an engine runs, the exhaust valves become hot. So if the fuel-air mixture is injected into the hot cylinder, and if that fuel just happens to be hydrogen, then the hydrogen is likely to ignite too early, giving rise to what he describes as "abnormal combustion."* Another issue relates to the comparative densities of gasoline and hydrogen, with hydrogen, of course, being far less dense (after all, it is a gas.) What this means is that in order to get the required level of fuel for combustion there needs to be a lot more hydrogen in the cylinder: 29.5% of the volume of the chamber versus 1.7% for gasoline. As a result, it is difficult to get a sufficient amount of air in a typical cylinder for complete hydrogen combustion.
However, in a rotary engine, the intake, combustion, and exhaust chambers are separate from one another. Consequently, when the hydrogen is injected into the rotary engine, there is no premature combustion. In addition to which, the air/fuel issue is more readily addressed by adding an injector to the rotary engine, which is exceedingly difficult to do given the packaging of a typical combustion chamber. These factors, as well as the longer cycle of a rotary engine, means that there is more complete combustion of the hydrogen, which means that there is better use of the available energy.
To be sure, Kashiwagi confirmed that a fuel-cell is more efficient than hydrogen combustion in a rotary engine. But there is one non-trivial factor that needs to be taken into account. Fuel-cells for automobiles are still an amalgam of a science experiment and fine jewelry creation. In the Mazda approach, the engine in an RX-8 Hydrogen RE is essentially the same engine that is used in an RX-8 that you can find on a dealer's lot right now. Consequently, the manufacturing cost for the engine is but a mere fraction of that of a fuel-cell. So ramping to production is readily accommodated.
But there's another advantage to using what is ordinarily a gasoline-powered engine. The comparative low density of hydrogen versus gasoline means that there needs to be two storage tanks to contain the 105 liters of hydrogen that are held at a pressure of 35 MPa (or 350 bar). These tanks are located in the trunk space of the RX-8 Hydrogen RE. This provides a sufficient amount of fuel for the vehicle to travel approximately 62 miles (a.k.a., 100 km). However, given that there could be some issues finding a hydrogen refueling station, and given that this is an internal combustion engine, Kashiwagi explained that the fuel system is engineered such that there is a 5-liter (1.3-gallon) gasoline tank on board, as well. With the flip of a switch mounted on the dashboard, the vehicle can be powered by gasoline. The amount of fuel is sufficient for about 28 miles of travel.
As Kashiwagi simply put it, "The hydrogen engine is very practical technology versus the fuel-cell vehicle."
*FYI: According to Mazda engineers, the flame front of combusted hydrogen travels at approximately 265 cm/sec in stoichiometric conditions. It is a comparatively slow 40 cm/sec for gasoline. Which explains, in part, why hydrogen is the fuel of choice . . . for NASA.
Can You Zoom Green?
While it may sound rather oxymoronic, since 2007, Mazda has been pursuing what it calls “Sustainable Zoom-Zoom.” After all, doesn’t the “Zoom-Zoom” part seem to undercut the “Sustainable” part?
That may seem to be the case, but not to Seita Kanai, Mazda’s director and senior managing executive officer in charge of R&D, Program Management and Powertrain Development; and president, Mazda Engineering & Technology Co., Ltd., and his staff. He explains that the basic policy is to provide “benefits to all Mazda owners, rather than limiting our efforts to a few eco-friendly vehicles.”
So they’re taking on a program that addresses both basic vehicle technology and manufacturing processes in order to achieve a 30% improvement in fuel economy by 2016—a 30% improvement versus where they were in 2009. This,
Kanai said, will require “a renewal of almost all of our powertrains and platforms.”
They are working on new diesel and internal combustion engines. The diesel will feature piezo injectors, a two-stage turbocharger, a new type of particulate filter, and an aluminum block. The gasoline engine will feature new direct injectors and variable valve timing for both intake and exhaust. What’s more, new six-speed automatic transmissions are being developed that, he said, will provide better fuel economy on the order of 4% to 7% compared with today’s automatics.
In addition to which, they are taking more than 100 kg out of new products. There will be a new vehicle platform in 2011 that has cut that amount of mass compared to current models, and then an additional 100 kg will be taken out by 2016. All Mazda products will be lighter as they are developed.
Kanai said that this is being accomplished in three ways: (1) creating more “ideal structures,” via CAE analysis; (2) using new manufacturing processes for forming (e.g., hot stamping; hydroforming) and joining; and (3) performing material replacements, such as increased use of aluminum, magnesium, and plastic. He stated that for the first 100 kg, he estimates that 84% is via the improved structural design (which, he noted, would also be less expensive to produce), 13% through new manufacturing processes, and 3% by material replacements.
Mazda, of course, has engineered a hybrid. This one uses the Hydrogen RE system that’s used on the RX-8 hydrogen vehicle, but it includes a series-type hybrid setup, as well. Called the Premacy Hydrogen RE Hybrid—the Mazda5 is known as the Premacy in Japan—the system transforms all engine output into electricity (the rotary engine is directly connected to a generator) and then uses that electricity to drive the permanent magnet synchronous motor to drive the wheels. There is a lithium-ion battery package that stores energy from the generator and the regenerative braking, then supplies it to the motor as required.