ron@hpfcmt.UUCP (ron) (04/16/85)
Re: Why water-cooled ?? Water cooling has a tremendous number of advantages in the longevity area. A properly designed and maintained system can allow tighter tolerance parts and a steadier operating temperature thus leading to better performance and less thermal stress on the machine. First, performance. Higher compression ratios typically mean higher output from the machine for the same displacement and weight. Part of the difficulty with higher compression (or turbocharging for that matter) is increased combustion chamber temperatures. There are numerous ways of dealing with the problem ranging from water vapor injection, to inter-cooling, to sodium-filled valves to more exotic materials. Still, the root problem is that a great deal of heat must be removed under a wide variety of conditions. Liquid cooling has a long history of doing this very well. Admittedly some tricks with oil sprays and oil cooling can help but the water jacket around the cylinder head is a life-saver for some hot parts. Second, longevity. A steady state operating condition is conducive to the longest engine life (makes intuitive sense). Liquid cooling has the ability to establish and maintain a steady state temperature (the biggest factor in 'steady state') under conditions ranging from the design upper air temperature assumption (probably around 100 F) down to below freezing. The point here being that engine operating temperature can be held constant while the outside air temperature can vary quite significantly. For an air-cooled engine this is absolutely not true. The engine temperature (cylinder head temp) will vary with: power output, airflow over the engine, RPM, outside air temperature and the amount of dirt on the cooling fins. Given that airflow changes with speed and outside temperature changes with time of day and season then the air cooled engine is operating in anything BUT steady state. (As a specially stressful case consider what happens when riding on a hot summer day -say 95 F- into a summer shower. The cylinder head may be running around 200=>250 or more in the hot air and then receives a quenching of 50 degree water spray when you ride into the shower. It's a wonder that there aren't more cracked cylinder heads occurring out there !) Airplane engines typically have their overhaul intervals reduced by such harsh treatment like this and are not very different from motorcycle engines. So, those are some simple advantages. There are disadvantages to water cooling too. 1. Increased complexity of the design due to the plumbing,pumping etc 2. Pumping losses due to the necessity to force water circulation. 3. Increased maintenance of the cooling system. (Change and flush coolant frequently for best results) 4. Lengthened warmup time due to the higher specific heat capacity of the engine. It takes longer to heat the metal-plus-water combination than the metal-only combination the operating range (This is the highest wear regime of the engine temp range => warmup) So, what's the best choice ? Naturally, that depends. Touring machines are commonly water cooled these days because their owners want and expect very long life (relative to motorcycles) and are willing to put up with the disadvantages to gain this. Banzai riders traditionally rode air cooled machines because the weight just wasn't worth it. (And besides, who ever heard of getting 75,000 miles out of a machine ridden so hard ?) The picture is becoming less clear now that machines like the Ninja 900 and the V65 Sabre are around. But the trend seems to be that water cooling is getting to be lighter in weight (with careful design) and so these 'performance' machines are utilizing the advantages of water cooling in their design. It's actually quite amazing. My 1982 air-cooled,chain drive CB900F actually weighs MORE than a friend's 1984 water cooled, shaft drive V65 Sabre. The bikes are intended for about the same type riding and yet the bike with the 'heavier' features weighs less. If I had to choose right now between the two obviously the Sabre would be my choice. No chain, no fuss. Water cooled, longer life. What more could you want ? Ron Miller H-P Ft. Collins Systems Div. Ft. Collins Colo. at: ... ihnp4!hpfcla!ron-m