Dan Gurney is no stranger to creating his own engine. In the 1960s he teamed up with Harry Weslake to create the Gurney-Weslake V12 for the Eagle TG1 Formula 1 car, with which he became
the first American driver to win a grand prix in a car of his own construction
(the 1967 Belgian Grand Prix).
(Photo courtesy AAR Archives.)
Clearly visible (above and below) are the twin counter-rotating crankshafts at the base of the Gurney MC4S motor. The crankshafts for this twin-cylinder engine are about half the length of an inline engineâs, and they sit in a one-piece cast aluminum sump. The head and sump are held to the block via seven through bolts.
In addition to a dramatic reduction in vibration and elimination of the engineâs fore-aft moment, the MC4S also features an optimized four-valve head design that improves airflow and power output to turbo engine levels on pump gas. (above and below)
Dan Gurney isnât just a legendary race driver, but a man who has developed some remarkable vehiclesâand engines. Like his latest project, the Moment Canceling 4 Stroke (MC4S) engine.
After reading the patent, slogging through its legalese, and ignoring the straight-cut gears shown in the patent drawings, itâs pretty obvious the 84-year-old Gurney is on to something. The MC4S is not only a perfect fit for motorcycles (and light aircraft), but for an increasing number of cars and light truck applications as well. Its natural balance and impressive estimated power figures point to a design that is just as at home under the hood of a sports or luxury car as it is next to the battery pack of a range-extended electric vehicle.
However, Gurney isnât about to get caught up in speculation. Though excited about the engineâs prospects, he admits itâs too early to make any definitive claims about the engine. âNaturally, weâd like to look at [the engineâs] long- and short-term viability, both materially and financially,â says Gurney. âAll offers to help are very much appreciated and will be considered. At the moment, however, our target is to prove that the simulation data is correct. We plan to be running by the end of this year on our test rig, and we expect to get confirmation in a reasonably short period of time after running.â He says that the development âwill take patience.â If all goes well, a further four engines will be built, though itâs not clear whether all will be built to the same exact specification, or be used to test other variants.
The MC4S as envisioned in the patent is a high-performance, dual overhead cam, liquid-cooled, oversquare (bore greater than stroke) motor with a one-piece cast aluminum sump. Seven through bolts attach the crankcase and sump through the engine block, much like Roverâs 1989-2005 K-Series four-cylinder engine. Unlike the Rover, which cost-reduced the engine to the point of failure, Gurneyâs engine has Nikasil-coated bores instead of liners and is fitted with a multi-layered steel head gasket. As the cutaway of the engine shows, however, this is as far as conventionality goes. From here on, it gets a little weird.
Though it looks like a U engine with geared counter-rotating cranks at first glance, the crankshafts are perpendicular to a line running through the pistons and camshafts. Odd-numbered pistons are located on one shaft (up to four for each, according to the patent), while even-numbered pistons are located on the other. The crankshaft gears determine the timing of the pistons to each other, and cancel the horizontal imbalance found in inline engines by reducing the transverse motion sent through the vehicle. This eliminates the forward/backward rocking motion common in inline designs, and significantly reduces vibration. Among the side benefits are improved bearing life, a reduction in the stresses sent through the vehicleâs structure, and the elimination of expensive vibration dampers. In addition, less vibration is sent to the short dual overhead camshafts, which are operated by gears connected to the crankshafts and a chain. A variable valve timing system controlled by the ECU is part of the package.
Each crankshaft is about half the length of a single crankshaft, and this contributes to the bottom endâs rigidity. The idealized MC4S seen in the patent uses ball bearings in place of shell bearings, though those could be used inÂ a less exotic variant. Further, with the odd and even cylinders located on different crankshafts, there is no need to offset the cylinders. This reduces engine size, and makes for simpler, more uniform water coolant passages.
Bore diameters range from 1.5 to 7.0 in. (38 to 178 mm) and strokes from 1.5 to 5.0 in. (38-127 mm) are mentioned with an oversquare range of 1.1 to 4.5 possible. The squish area for the flat-top pistons ranges from 24% to 35%, with a preferred area of 31.5%. With this squish area, the preferred mean piston speed is less than 4,200 feet per minute.
By examining more than 200 valve configurations, Gurney and his team discovered that small changes in the intake and exhaust angles had huge effects on output. Specifically, a 15% increase in the cubic feet per minute delivered high velocities: 0.5 to 0.95 Mach. (Most naturally aspirated engines have a velocity below 0.4 Mach.) Each pair of intake valves has a combined area (in square inches) of 28% to 38% of the bore, while the same measurement for the exhaust valves is 14% to 20%. In addition, the intake ports have a combined area of 42% to 65% of the valve area, with 53.4% seen as ideal. Intake ports are set at 7.9Â° to the left of vertical, while exhaust ports are angled 8.4Â° to the right of vertical.
Three intake and exhaust port designs were created for the MC4S: straight, improved and optimized. Both the improved and optimized designs flowed more air, but the optimized design showed significantly more area in terms of flow versus valve lift, and a consistently greater volume than the straight configuration. The improved port configuration is an intermediate step, with a Venturi shape to the intake port and a gradual curve to the exhaust port. However, it is the way in which the features of each are interconnected that produced the experimental results Gurney and his team hope to replicate in the metal before the end of 2015. Critical to the design of the intake ports is the shape of the inside radii, which forms a Venturi that allows the flow boundary layer to follow the shape of the port, and reduce pressure in the pipe in order to increase flow. Almost as critical to flow are the valve seat and valve seat undercut angles. For the optimized design, the preferred intake valve seat angle is between 48Â° and 52Â° with a valve seat undercut of 38Â° to 42Â°. For the exhaust valves, these numbers are 40Â° to 52Â° and 45Â° to 65Â°.
Compression ratios from 9:1 to 10.5:1 were investigated with the optimized porting system, and simulations showed a BMEP (brake mean effective pressure) more than 100% greater than found on most naturally aspirated engines. It also showed that the effect of increasing the compression ratio leveled off, leading to the conclusion that the MC4S engine can produce high power outputs with a lower compression ratio. This reduces stress on the engine, and results in a power loss of less than 2.0%, according to the simulations. These also show that a 1.8-liter four valve with straight porting should produce 141 hp at 8,000 rpm when running a 9.5:1 compression ratio. Moving up to the optimized porting increases this numberÂ by 85%, giving a theoretical power output of 262 hp on pump gas. Like his friends and colleagues say, Gurney is on to something, and it doesnât need turbos.
With its transverse counter-rotating crankshafts and perpendicular camshafts, intake and exhaust, the MC4S would appear to be well suited for installation in a motorcycle. The rigid sandwiched block construction, compact integrated gearbox, rotational moment cancellation, significantly reduced vibration, and porting along the bikeâs axis mean it should package well and eliminate many of the problems that plague current larger displacement inline or V-twin engines.
In addition, it would make an ultra-low vibration/high efficiency engine for a range extender EV. It would be relatively easy to adapt the MC4S to a transverse front-drive âend-onâ gearbox or a rear-drive transmission. It might even be advantageous for reasons of cost, vibration, rigidity and crashworthiness to keep the perpendicular twin cylinder head design, and replicate it for any additional pairs of cylinders. (A mode canceling eight cylinder would be amazing with an intake-exhaust/exhaust-intake/intake-exhaust/exhaust-intake head layout and turbine-like smoothness.) Then again, mating an optimized port twin cylinder design with a 48V electrical system might be all thatâs needed to produce a powerful, quiet and smooth hybrid powertrain. The possibilities, like Dan Gurneyâs fertile imagination, are endless.Â