Manic Motors
Here we present another strange engine configuration. As though the “H” engine wasn’t complex enough, some aero-engine folks decided coupling it to a turbine would certainly spice things up!
If two crankshafts would be cool, how about adding a turbine… or even a jet? That would be the ultimate cool. Wouldn’t it?
Article by Doug Switzer Moto/ology Publisher
In our last issue, we looked at how reciprocating piston engines grew in size and complexity with the addition of more and more cylinders. To facilitate the addition of more cylinders, engines became more and more complex with V8, V12 and V16 configurations. They even adopted double crankshafts to accommodate two flats of 8 or 12 cylinders, enabling some designers to produce hilariously complex “H” Engines with 16 and even 24 cylinders turning two crankshafts driving a common output shaft. Ultimately, these contraptions started to test the limits of reliability and with the advent of the turbine engine, things became greatly simplified and much saner.
The gas turbine or “jet” engine as we have come to know it was a revelation and enabled a giant leap ahead in aircraft design and performance. However, the “old school” designers of piston/propellor-driven aircraft were still prone to experimenting. With the reliability of the early jets still somewhat questionable, these folks decided they’d harness the best of both worlds and try something quite outrageous.
The idea of hooking up a jet to a piston engine was born. Now, we should point out that this wasn’t exactly a totally new idea, In the 1930s, the Italian designer Secondo Campini joined up with the Caproni aircraft company and built a piston/jet hybrid engine and flew it in an aircraft during the early stages of WWII. In the Caproni/Campini design the piston engine drove a compressor that fed compressed air to the jet. This configuration became known as a motorjet and the result was less than optimal as the performance of this strange hybrid wasn’t really any better than it’s contemporary piston/propellor designs. While not a turbo-compound engine, the motorjet bears mentioning as it is another form of “hybrid” powerplant. Unfortunately, this arrangement simply didn’t produce enough thrust. With the success of the German turbojet-powered aircraft of the time, the project was abandoned.
The complex and intricate piston aero-engine designers however were not about to give up and several firms started looking at the concept of a piston/turbine “compound” engine. In this design, a turbine is coupled to a piston engine and uses the exhaust from the piston engine to augment the flow of the “jet” portion of the powerplant and improve fuel efficiency. These exhaust gases are routed through a turbine and released for additional reactive thrust like a normal jet. They were, however, also coupled to a drive system allowing additional power to assist in turning the propellor of the piston engine portion of the device, or in some instances, assist in driving a second “contra-rotating” propellor.
Needless to say, this all sounds pretty complicated, and it definitely is! So much so, it’s quite difficult to actually define any advantages to these engines, let alone make a case for their development. So, after only a few experimental R&D flights from various manufacturers, the idea was abandoned.
So, who were the major players?
Our old friends at Napier in England became proponents of the compound engine and the development was also taken up by firms in the Soviet Union and several other countries. The first iteration from Napier was an impressive contraption featuring an H-24 engine similar to the Napier Sabre that was flown successfully in the Hawker Typhoon and Tempest fighter-bombers of WWII. However, this set-up was to be a diesel and proved to be overkill. It was also ridiculously complicated, so the H-configuration was abandoned in favour of a more manageable flat 12-cylinder diesel piston engine coupled to a turbine that was mounted on its underside. This device was christened the Napier Nomad and it flew in test flights mounted in the nose of a modified Avro Lincoln Bomber.
Several other designs that used the turbo-compounding system were developed, but the Wright R-3350 Duplex-Cyclone is probably the simplest and most efficient of the type although early versions suffered from overheating and reliability issues. Further, it is said to be the only compound design to go into large-scale production and use. The Wright R3359 was used in several military and civilian aircraft including the Boeing B-29 and Consolidated B-32 bombers and the Lockheed Super Constellation airliner and Fairchild Flying Boxcar transport aircraft along with many others.
The Russians also got in on the turbo-compound engine bandwagon with their independently developed efforts such as the Dobrynin WD-4K that was used in some of their early post-war strategic bomber and transport aircraft.
As far as aircraft were concerned, a lot of money and time was spent on the development of these turbo-compound powerplants between 1944 and the late 1950s, but the rapid pace of jet engine development led to them being dropped in favour of the much simpler pure jet and turbo-prop designs.
The idea of turbo-compounding engines is far from dead however and the technique is used to this day in some diesel truck engines such as the Detroit Diesel DD15. We must point out that these engines are considerably less complicated than the astounding aero engines we’ve [presented here.
We should also touch on the development of a related type of turbo-compounding used in the diminutive Formula One race engines of the current era. Since 2014, these 1.6 litre V6 engines have used a compounding type of energy recovery unit to reclaim exhaust energy and generate electricity that is stored and used in auxiliary electric motor systems installed in the main drivetrain. These very sophisticated powerplants also use several other energy recovery systems to augment their power and when used together, these small-displacement engines produce up to 1000+ hp while screaming along at more than 17,000 RPM!
In addition to the exhaust recovery system, the cars also recoup energy from onboard motor/generators that manage and draw power from braking and accelerating. The electrical storage and release of the energy involved is immense and can be dangerous to drivers and crews, so a system of warning lights is used to display the status of the energy recovery systems.
The full story of the design, development and efficiency of these engines is beyond the scope of this article so we’ll see if we can delve into it deeper sometime in the future.
Watch for more Manic Motor articles in upcoming issues of Moto/ology!