We look at three interesting variations on the venerable internal-combustion engine…
The burble of a V8, the scream of a rotary, or the ring-ting-ting sound and smell of a two-stroke … petrolheads love every aspect of internal-combustion engines (ICE). Yes, they may be a dying breed and could even be phased out in our lifetime, but before electric powertrains take over, it’s worth honouring the impressive efficiencies and innovative thinking that has developed ICEs from very humble beginnings into power sources with long-range and quick-refuelling capabilities.
Why ICEs are so popular
Petrol and diesel contain vast amounts of chemical potential energy (more than 40 MJ/kg), playing second fiddle only to nuclear energy when it comes to energy density. The key is how to best transform this chemical energy to a motive force, and that’s something reciprocating piston engines are able to do with relatively high efficiency. It’s best to approach engine design from an energy point of view, because the energy in the fuel supplied to any engine is a known quantity and you can measure the output in terms of power (which is a function of torque and engine speed).
Therefore, you can calculate the efficiency of any design by dividing the useful output energy by the amount of chemical energy. In a process you are no doubt familiar with, during an ICE’s combustion process, the pressure rises dramatically in the combustion chamber and this pressure pushes the piston down the cylinder (in a piston engine). This then increases the volume, which results in work being performed. Now, over the years, there have been some particularly innovative approaches to improve the basic ICE principle and here we highlight three particular examples of out-the-box thinking.
1. Four-stroke rotary engine
The design of the first rotary engine is attributed to German engineer Felix Wankel in 1929, but it was German car manufacturer NSU that really spurred on its development in the 1950s. An agreement with Mazda in 1961 allowed the Japanese firm to further advance the technology, with the engine powering the Cosmo 110S in 1967. Compared with a piston engine, a rotary has fewer moving parts, is lighter, smoother and produces more power. The main problem, however, was apex seals (they function like the rings of a piston engine) that tended to wear out quickly. With its 2002 RX-8, Mazda both solved this and improved the fuel efficiency (a rotary engine’s Achilles’ heel), with the Renesis rotary engine seeing the ports moved to the side of the rotor. The engine won the International Engine of the Year award in 2003.
A single-cylinder, four-stroke engine delivers a power stroke for every 720 degrees of crankshaft rotation (two full rotations). In a Wankel engine, there are three power strokes per rotor rotation but, because of the one-to-three ratio, there is only one power stroke per crankshaft rotation. For comparative reasons, therefore, a Wankel engine’s capacity is normally doubled to compare it with an equivalent naturally aspirated, four-stroke piston engine. In the case of the RX-8, you can compare the engine’s performance to a 2,6-litre, naturally aspirated piston engine. Unfortunately, Mazda discontinued the Renesis engine in 2011, as it no longer met emissions standards, but there have been ongoing rumours that we will see a next-generation Wankel rotary engine powering the new RX-9, rumoured to launch in 2020.
Example: 2002 Mazda RX-8
Cubic capacity: 308 cm3
Piston-engine cubic-capacity equivalent: 2 618 cm3
Compression ratio: 10,0 to 1
Valvegear: no valves, intake and exhaust ports
Max power: 170 kW at 8 200 r/min
Max torque: 211 N.m at 5 200 r/min
Fuel cons: 11,3 L/100 km
0-100 km/h: 6,4 sec
Maximum speed: 235 km/h
2. Four-stroke piston engine
What makes this powertrain special is its oval pistons (they’re actually oblong-shaped with rounded corners). They allow extra valve area and increased gas flow through the combustion chamber, which in turn results in higher power outputs. The NR 750 engine, with its eight valves, two conrods and two spark plugs per cylinder, is a complicated design that originated in the late 1970s when Honda went Grand Prix racing. The regulations back then allowed
500 cm3 machines with either two-stroke or four-stroke engines. Because two-stroke engines produced nearly double the power of a similar-capacity, conventional four-stroke engine, Honda’s engineers came up with a way to increase the airflow through the naturally aspirated engine, allowing it to rev higher. The oval-piston engine was born.
The 500 cm3 V4 engine in the race bike could rev to more than 20 000 r/min and produce around 100 kW. Unfortunately, this was not enough to be competitive in GP racing, although the bikes did enjoy some success in endurance formats where the four-stroke was more efficient than the two-strokes and could stop less often for fuel. Oval pistons were also very expensive to make and, in the end, the extra cost did not justify the theoretical advantage. Interestingly, in the 1990s, Honda produced a limited-edition (700 units) road bike – the NR750 – featuring this technology. It was priced at a then-astronomical £38 000. Needless to say, they are worth a lot more than that now.
Example: 1992 Honda NR 750 motorcycle
Cubic capacity: 748 cm3
Compression ratio: 11,7 to 1
Valvegear: d-o-h-c, eight valves per cylinder
Max power: 92 kW at 14 000 r/min
Max torque: 66 N.m at 11 000 r/min
Fuel cons: 7,96 L/100 km
Maximum speed: 257 km/h
3. Turbine engine
Interest in turbine-engined cars can be traced back as early as 1927 with Opel and later in 1939 with Rover. The most famous production-ready turbine vehicle was made by Chrysler, which built 55 aptly named “Turbine” cars that were handed out to the general public for real-world evaluation purposes. A turbine engine’s advantage is that it is relatively simple, compact, lightweight and powerful. On the negative side, it is very inefficient when not at optimal speed and load conditions, and this was the main reason why turbine road cars never took off.
Racing is different, however, and here turbine power enjoyed more success. The most famous racecars were the Paxton STP Indy cars and Howmet TX (pictured) that won two Sports Car Club of America races in the 1960s, becoming the only turbine racecar to ever win a race. The turbine engine was sourced from Continental after the company had no need for the TS325-1 unit that was meant for a (subsequently aborted) military-helicopter programme bid.
Apart from high fuel consumption, turbine engines have another great disadvantage: slow response times. It could take more than three seconds for the turbine to respond to an acceleration request, although this was solved by using a wastegate to dump the energy not needed, but keep the turbine “on the boil”. At least the driver did not need to worry about gear shifting, as the turbine’s speed range was wide enough to cover the racing application with only a single-speed transmission. Unlike a possible Wankel rotary renaissance, it is unlikely automotive turbines will make a comeback. Noise regulations, excessive fuel consumption and emissions will see to that. There have, however, been proposals to use a small turbine as a range-extending generator in an electric vehicle … time will tell.
Example: Howmet TX turbine racecar
Type: Continental TS325-1 two-stage gas turbine Piston-engine cubic-capacity equivalent: 2 960 cm3
Max power: 260 kW
Max engine speed: 57 000 r/min
Engine mass: 77 kg
Fuel tank size: 120,0 L
Fuel: Jet A
Maximum speed: >290 km/h
Race wins: 2, Sports Car Club of America (SCCA)
Author: Nicol Louw