Liquid-cooled engines appear to be entering the mainstream at Harley-Davidson, as it introduces two significant new powertrains for 2014. One of them is the new Revolution X family of modular 500- and 750-cm3 engines that will power the new “Street” range of global bikes aimed at younger, urban-dwelling riders. The other is a clever application of “strategic” liquid cooling in H-D’s traditional Twin-Cam 103 engine series.
Both liquid-cooling applications are testament to the increasingly stringent global noise and emissions regulations facing the motorcycle industry, which powertrain engineers believe will steadily make strictly air-cooled engines obsolete.
The Revolution X is Harley’s first liquid-cooled production engine since the Revolution (V-Rod) family debuted in 2001. Unveiled at the recent EICMA motorcycle show in Milan, Revolution X are 60° V-twins with cast-aluminum crankcases, cylinder barrels, and heads. Six-speed transmissions are unitized with the vertically split lower-end assemblies, and final drive is by reinforced cogged belt.
The 60° cylinder splay (also used on the V-Rod) was selected to minimize the overall height of the wet-sump engines and thus maintain the low seat height that many riders prefer, noted Chief Powertrain Engineer Jeff Coughlin during Harley’s Milan press conference.
Common stroke shared
The cylinders feature pressed-in iron liners. The 500 and 750-cm3 versions share a 66-mm stroke; the 500’s bore is 69 mm, while the far more oversquare 750 has an 85-mm bore. Valvetrain architecture in both engines includes single-overhead camshafts driven from the crankshaft by roller chain and operating four valves per cylinder through roller rocker arms with screw-type lash adjusters.
A single gear-driven balancer shaft helps keep vibration in check; the Revolution X engines are designed for safe 8000-rpm operation. Although official peak power and torque figures were not available when this article was published, a U.S. H-D dealer service tech told AEI the 750-cm3 Street model will be rated at 55-60 hp (41-45 kW) and 42-44 lb·ft (57-61 N·m).
The 2014 Revolution X engines and 500/750 Street motorcycles will be produced at H-D plants in Kansas City, MO, and Gurgaon, India, for global sale.
“Twin-Cooled” Twin Cam 103
Adding liquid-coolant galleries around the exhaust valve seats of the air-cooled Twin Cam 103 engine family, with a pair of fairing-mounted coolant radiators, thermostat, electric coolant pump, and their attendant plumbing (see accompanying CAD illustration), was one of many technologies included in Harley-Davidson’s ambitious “Rushmore” development program to upgrade its large-displacement touring and cruiser models for 2014.
H-D calls the hybrid air/liquid cooling system “Twin Cooling,” and it’s only available on certain high-output variants of the Twin Cam 103 family (the Ultra Limited and Tri Glide Ultra models, specifically). Engineers set out to increase peak torque over a broader operating range by increasing compression ratio—in this case from 9.7 to 10.1:1. The end result was a 10.7% gain in peak torque versus the standard air-cooled Twin Cam 103 unit, according to the company, but it required employment of a “strategic” cooling strategy for which Harley had filed patents in 2011.
The system circulates liquid coolant through the cylinder heads (which retain their external fins so revered by Harley traditionalists) in a passage around the exhaust valve seats; the coolant then circulates through a pair of air-to-liquid heat exchangers mounted in the bikes’ right- and left-side fairing lowers.
The Twin Cooling system naturally adds weight and complexity to what had been a fairly simple machine, but the benefits include lower and more consistently even cylinder head temperatures. It is not yet clear whether rider and passenger will also enjoy reduced heat during summertime operations—most critically in stop-and-go conditions.
As AEI has reported, other motorcycle OEMs recently have employed “strategic cooling” in their air/oil-cooled engines, including BMW in its “boxer” models (in the upper cylinder area around the top piston ring lands and in the lower flanks of the cylinder heads around the exhaust valve seats), and Honda in its CB1100 introduced in 2013. The inline four-cylinder Honda features oil galleries neatly located around the spark-plug area of the cylinder head.