I'm really having a time trying to find information on how long period Dwell times (from long rod-stroke ratios) are influenced by low piston speeds from a short stroke motor. For example building a small displacement 10k+ rpm motor with a resultant piston speed of 65 feet/sec.....with a rod-stroke ratio of WAY over 2:1...nearing 3. how dows this low speed influence the dwell time? just increase it dramatically? improving the fill/empty time and ability?
What would be the difference with a turbo/SC'd engine? any?
"Looks are much more important now. It dosen't matter that Brittney Spears has nothing to say and is about as deep as a birdbath..." --David Crosby
thats not the answer...
the firstly mentioned conbination and how that either enhances a forced induction engine or degrades a forced induction engine. im asking about how low piston speed in a high rpm engine affects extremely long dwell times and its ability to fill/empty and if a forced induction engine eliminates or is enhanced by this design. that was my question.
The length of the stroke does not affect the dwell time. The rod's length in relation to the stroke does. The shorter rod will have less 'dwell' time at tdc, but the long rod will have greater acceleration (not to be confused with velocity) back down towards bdc.
The longer rod engine will have less time to fill the cylinder due to the fact that while its piston is still hanging around at TDC, the short rod engine has already beguns its travel (and suction) downward. This is advatageous for low to mid engine speeds. Once the short rod engine reaches high RPM (street driven high rpm i consider 6k rpm-up) The stress of that short rod pressing against the cylinder sidewall (termed: sidewall load) is not worth its ill effecs. (cracking sleeves, overheating, power loss due to excessive friction.)
In racing applications 10 out of 10 times you go with a short stroke long rod engine, unless there is some maniacal 6k and below rpm limit. Your question of piston speed is merely a factor of the length of the stroke itself. With todays current metallurgy standards, in a racing application with top notch internals, i would still try to keep below 4500fpm mean piston speed.
people arent reading the whole post aparently...i mentioned rod-stroke ratio...thats the relation of the rod length to the stroke length....etc.....this is a known. its something i know about already. it makes a long period dwell time. this is a good thing. i know abou that already. im trying to find ou the influence ON dwell time of the low piston speed. 90 feet/sec. (5400 fpm) is the point where friction spikes and you get very little return on exceeding that. That is not the issue. friction is not hte issue. im looking at piston speeds of around 60 fps (3600fpm) and thats at 10k rpm or so.....and thats what i was wondering. what effect on dwell time is there FROM low piston speed. is there any at all...AND...if so is it increase it any or decrease it any and if not then fine.
It's a well established fact that, kinematically, a longer rod ratio produces a more uniform piston velocity throughout its stroke. Thus a long rod produces lower peak piston velocities/accelerations/jerk etc. Generally, intake flow stagnates at sonic velocity. And since intake velocity through the ports is relatively proportional to piston speed, intake flow benefits from reduced peak piston velocity.
Of course engine design, especially design variables like stroke/bore ratios and rod ratios, is an excercise in compromise. For low speed engines, a long rod is especially beneficial. It reduces peak piston velocities/accelerations and thus inertia loads on the rod and crank bearings. A long rod also minimises rod angulation and thrust load/friction losses at the piston skirt. Take a look at a commercial diesel engine, they use as long a connecting rod as they can physically fit within the engine. Unfortunately, long rods are also heavy.
For an F1 engine, inertia loads (at 18,000 rpm) are the primary concern. So a very short, lightweight rod is used. In fact they use as short a rod as possible to minimise weight. And the rods are usually titanium.
