With a promise of increased mechanical efficiency, greatly reduced fuel use and the ability to be 'tuned' for power or torque by altering the profile of key internal components, the Revetec engine offers plenty of fresh ideas for automotive technology in the next few years. Coupled with its modular assembly, the Revetec engine could possibly change the direction of internal combustion engine development in the not-too-distant future.
At first glance, the engine (or the one prototype that has been built so far) looks not unlike any other engine churned out by Subaru, VW or Porsche over the last half century. Four cylinders, opposing in a flat arrangement with a flywheel at one end and accessories drive at the other. But a closer look at the engine in pieces, or a scan of the blueprints, reveals an engine that lacks a crankshaft in the true sense of the word.
Background to its Development
Brad Howell-Smith explained to me how he came up with the Revetec principle. It sounds like a pub yarn, but he swears it's true.
"It came to me in a dream," he admits. "Just like that - in my sleep.."
The look on his face convinces me he's not joking.
"The first prototype took me six months," he explained. "It was built on a drill press and a $2000 lathe I had at home."
That was in 1995/96. Originally from Sydney, home at that time was on Queensland's Gold Coast where Brad was working as a mechanic for a Holden dealership.
"The first thing I did was go to Queensland Institute of Technology. I'd come up with the concept but I wanted someone else to verify what I was doing. I presented (the idea) to them and, first off, I said that I was going to increase the level of thermodynamic efficiency beyond 50 percent. They laughed at me. They laughed, so I said 'well, I'll prove it to you.'"
How it works
In terms of its 'top end' the Revetec engine uses conventional technology of cam-driven poppet valves and a piston sliding up and down in a bore. It's what happens underneath the piston - and to a lesser extent, the rate at which these things happen - that the Revetec differs.
Rather than a conventional crankshaft and connecting rods, the Revetec engine uses a system of rollers and lobes, not unlike a like a conventional roller valve train arrangement, to transfer the back-and-forth motion of the pistons to round-and-round of the output shaft. Where the gudgeon pin of a piston/rod assembly would normally sit below the crown of a piston in a conventional engine, the Revetec engine employs two roller bearings side-by-side.
The rolling surface of each of these bearings runs against a three-lobe (trilobate) cam. There are two of these cams geared together on a co-axial output shaft so they rotate in opposite directions. The piston goes up and down like in a normal engine, but instead of a connecting rod applying a load to a crank, the rollers on the pin of the piston spin the lobes to create an output torque.
Each piston is locked to its opposing piston - the Revetec is necessarily a 'flat' or 'boxer' design - so in fact, each module of two pistons operates on the lobes - each lobe has two bearings rolling against it.
It's as simple - and as complex - as that.
What makes it potentially better than what we have?
The Wankel rotary engine - as formatted by German Felix Wankel and refined by Mazda, the only car manufacturer to put significant effort into its development - is the only engine that has come close to challenging the conventional piston engine for cost, efficiency and useability. And despite Mazda's best efforts, the rotary remains costly, inefficient and almost unused. The rotor motor almost destroyed Mazda during the 1970s as it threw millions of dollars at engineering to overcome its endemic problems with combustion sealing, cooling of apex seals, horrible emissions, difficult lubrication and fearsome fuel consumption thanks to its inefficient combustion chamber shape and high surface-to-volume ratio. The Revetec engine appears to combine the major benefits of the rotary - smoothness - with the proven technology and surface-to-volume aspects of a piston engine.
According to Howell-Smith, a typical thermodynamic efficiency level for an internal combustion engine is around 25 percent. That means around one quarter of the energy present in the fuel consumed by the engine actually makes it to the flywheel. The other three-quarters is dissipated by the cooling system as heat derived from lost heat of combustion, friction, incomplete combustion and mechanical losses.
"Frictional losses aren't really as great as what you'd expect. It's mostly all rubbing forces and you can measure the heat loss of the engine and the losses from the valvetrain. You have a big void of where those losses have gone. And when you look at all that, the void calculation; it's all mechanical inefficiency, obvious inefficiencies in the transfer of the power."
"The cylinder (shape) is the best, most efficient way to hold combustion - apart from a sphere - with the surface area considerations. But you can't move anything [like a piston] in a sphere so the cylinder remains the best compromise. The crankshaft is only 64 percent efficient; graph our design and it's 87 percent efficient, with the cam profile I picked. With a different cam profile it's possible to go to 90 percent."
In layman's terms, that means more of the juice extracted from the bang in the combustion chamber gets to the flywheel of the Revetec engine.
The gains are made in the relationship of the roller bearing to the surface of the crank lobe - compared to a piston and rod bearing - and the leverage this exerts on the crank. A conrod's fluctuating angularity while the piston moves up and down alters the effective length of the leverage arm the piston is working against. It's only ever at its maximum push just before the piston is halfway down its stroke - the remainder of the stroke the piston is not pushing against the crank with maximum leverage.
This especially true at the beginning of the stroke when combustion chamber pressure is highest and leverage is lowest; exactly when it should be at its highest!
The relationship between the roller bearings and the cams means Revetec engine has a more effective lever arm over a far longer period of stroke, In fact, the piston can apply maximum push on the piston at the beginning of its stroke and can still be pushing hard with the piston almost at the bottom of its stroke, sucking more power out of the combustion event.
The profile of the crank lobes determines the power and torque delivery characteristics of the Revetec engine. Changing the shape of the crank lobes can allow the piston to stay at top dead centre for longer. And of course, the opposed linked-piston layout means the piston must dwell at the bottom of its stroke for longer, too.
"That makes it especially relevant for two-strokes. Holding the piston down the bottom gives you better cylinder fill. By altering the speed and acceleration of the piston, you can control the porting a lot better than you can now."
The piston-down-longer situation also opens up new opportunities for, for instance, forced-induction engines. More intake charge can be introduced into the cylinder. Longer cylinder fill time reduces peak airflow demand on intake ports, meaning they can be smaller. Smaller ports, as well as allowing more freedom in aspects of head design such as coolant passage, introduce high turbulence for cleaner burning without the need for high engine speeds.
"You can also use a different acceleration characteristic on the down stroke to the upstroke by using asymmetrical lobes. You don't have to run symmetrical lobes."
But surely, asymmetrical lobes would lead to horrible balance problems, one of the key qualities of the Revetec engine being lack of vibration thanks to the contra-rotating layout of the lobes?
Brad agrees. "You can't go too far with that until you have balance problems, but balance shafts (to counter these undesirable vibrations) in cars are normal these days."
Another characteristic that different lobe shapes give the Revetec engine is mechanical advantage on the compression stroke. As Brad explains it, the lobe profile can be designed to get the piston travelling up the bore fast, then slowing it to increase the mechanical advantage to 'squish' the intake charge near the top of its stroke. That reduces shock loading on the piston and ring assemblies, allowing them to be lighter in construction for lower mass.
And that's not the only advantage according to Revetec. There's a tangible gain in drivability and throttle response to be expected, too. "If you look at a normal engine and the shock of compression and power application, you need a certain size flywheel to dampen the pulses. Our engine is smoother and requires less flywheel weight."
Lighter pistons and ring packs mean that new materials can be investigated - for instance, ceramics could be used as piston material. The pistons, being locked together 'as one' across the output shaft and without the sideways action of a crankshaft and con rod, don't need the added weight and height of skirts to stop them cocking in their bores. Ring packs can be mounted very high in the piston to reduce the emissions and detonation dramas of hot gas being left behind between the piston and the bore above the piston rings. And the lack of piston cocking means the ring seal isn't compromised.
All the major rotating masses spin in the one direction in a conventional engine. So when the engine is accelerating, it moves in the opposite direction to crank rotation as the inertia of the spinning components react against its mounts. In the Revetec design, all the weights spinning in one direction can weigh the same as all the weights spinning in the other direction. The result? No torque twist. Everything cancels out.
In the automotive arena, that leads to less NVH and lessens the reliance on large rubber or hydraulic engine mounts. In industrial applications, it reduces the characteristic of 'cradled' portable engines (such as small water pumps) jumping around, especially under varying speeds and loads.
The straight power output shaft can be made hollow which opens up new opportunities for, say, aircraft use. Mechanical or hydraulic control systems can be fed up the centre of the engine's to control variable pitch props in light aircraft.
Within the one crankcase, and using the same bore and stroke, different sized rollers and lobes can be employed, to dramatically alter the rev range over which torque can be delivered. In other words, the engine can be 'tuned' to deliver maximum torque at whatever combustion efficiency works best. That makes the engine attractive to hybrid vehicle use - the engine can be specified to get the absolute most from every drop of fuel, at relatively low revs, to drive a generator to power the vehicle.
That's the theory - now for the practice.
What's next?
During the second half of 2002, a batch of prototypes will be manufactured for continuing testing and development. The prototype you see pictured here was a runner, machined from aluminium by Revetec with some components such as cams and piston rings sourced from outside suppliers - in fact, the rings are Suzuki. It operates on a test-bed, for durability, lubrication and engine management purposes. The company is currently planning to test the engine in the US at Stanford by the end of this year, at which time will release unquestionable test figures. "The engine wasn't built for cosmetics, although the engine is easy on the eye, which is a good marketing tool."
After initial testing the prototype was stripped down and inspected. No abnormal wear was found in any component. The engine received a dozen pre scheduled upgrade modifications and two unscheduled mods. These were to the cooling system, and the raising of piston rings closer to the piston crown for adjustment of heat dissipation.
The next batch of engines - to be manufactured with more cast components, rather than machined from billet like the first one - will incorporate several design refinements, such as replacing the four-rail piston sliders with simpler two-rail items installed between the two crank lobes. The heads, too will be modified. These units will be extensively durability and dyno tested in China and Australia and used to further refine the technology and manufacturing logistics.
Originally, the concept of the Revetec engine concentrated under the pistons, but the engine's 'tuneable' piston speeds and its low idle speed has opened up a whole new dimension in cylinder head airflow. The huge range of speeds and the flow characteristics of the Revetec engine means there's plenty of forward planning happening. Like the also-Aussie Sarich Orbital engine of the 1970s that actually spawned a new generation of fuel injection technology, the Revetec project may cause the first wholesale rethink of induction systems since variable valve timing.
Revetec is looking at rotary-valve heads and a new design of head that has a scotch yoke arrangement that uses a short-stroke piston forming the ceiling of the combustion chamber that opens up ports, not unlike the ports in a two-stroke. This moving combustion chamber design - for want of a better term - could also be used to give the combustion charge an extra squeeze just before ignition, raising the effective static compression ratio for better thermodynamics.
"There's a lot of technology out there that we have exposure to," says Brad. "Anybody working with new engine technology approaches us, so we get to look at a lot of technology before anybody else does. People tend to relate to what we're doing."
"Mate, I could spend two days talking about this engine!"
Check out more at:
www.revetec.com
