crankcase compression ratio

Discussion in 'Performance Mods' started by jaguar, Nov 6, 2011.

  1. jaguar

    jaguar Well-Known Member

    Using the painstakingly accurate method of determining the crankcase compression ratio I outline at http://www.dragonfly75.com/motorbike/PCR.html I discovered that my reed valved 55cc has a ratio of 1.2 and that a meager ratio of 1.11 is existent with my 48cc piston port engine. This ratio is the ratio of compression of intake charge before its released into the cylinder thru the transfer ports. A higher ratio indicates there is more pressure at the time of transfer opening. 1.2 is for low rpm engines and up to 1.5 for high rpm engines. The reasoning is that at low rpm you don't want the intake charge blasting (via high pressure) into the cylinder too fast and looping around and exiting the exhaust port. But at high rpm you do want a fast transfer since there's so little time for the movement of charge to take place. A ratio of 1.1 is around 1/2 the pressure that 1.2 produces and is too low.
    So how can we increase the ratio? By reducing the space that contains the intake charge being compressed. This space includes the space between the two halves of the flywheel, the space under the piston, and the space in the transfer channels. To reduce the space between the flywheel halves we'd have to take apart the flywheel and bolt on aluminum spacers. Too much work for me baby! JB Weld can be used in the other two areas but increasing the piston weight would probably imbalance the crank and cause more vibration unless you drill out the center of the piston pin to counteract. JB Weld can be safely put along the walls of the transfer channels since the channel cross sectional volume is much more than the area of the transfer ports when opened.
    What effect can this have? It can increase the delivery ratio of intake charge into the cylinder which increases power throughout the whole rpm range.
    Since the total volume that can be added to the transfer channels is small and I don't want to add flywheel spacers, then I'm going to also add JBWeld under the piston and let you know how it goes. (I am well aware that temperature is pretty high under the piston crown which may disconnect the JBWeld there, leaving the side adhesion in effect.) This isn't new by the way. I have read of others doing this. Here's a graph showing the relationship of compression ratio to psi pressure:
     

    Attached Files:

    • CPR.jpg
      CPR.jpg
      File size:
      38 KB
      Views:
      562
    Last edited: Nov 6, 2011

  2. jaguar

    jaguar Well-Known Member

    Fiddling around, I found that putting the JBWeld in the piston would occupy only 15ml, and probably the same for the transfers. 80ml total reduction is needed to get the ratio to 1.2 which means the flywheel would have to have spacers (at least 2mm wide each) installed. rats!
    The problem exists with the 48cc piston port engines because the movement of the piston from closing the intake port to opening the transfers displaces only 23ml (23 square cm). Installing a reed valve greatly improves the ratio because then the measurement of piston displacement begins from top dead center.
     
    Last edited: Nov 7, 2011
  3. jaguar

    jaguar Well-Known Member

    My reed valved 55cc engine has a 1.2 ratio but only because the reed valve is located inside th intake tract and almost touches the piston.
    I have decided to pull the crank and use JBWeld to fix two aluminum plates in place.
    Graham Bell had wrote "all things being equal, a higher crankcase compression ratio increases transfer flow and peak horsepower".
     
  4. jaguar

    jaguar Well-Known Member

    Calculating I get these figures:
    A plate that is 1mm thick and 10.5cm in diameter occupies 8.65cm2 (square centimeters, millilitres). Looks like 3mm on each inside of the flywheels is a safe amount to add which would take up 52cm2. That would leave 148cm2. (148+23)/148=1.15 ratio
    Using JBWeld in the transfers might be enough to get the ratio up to the desired 1.2
     
  5. jaguar

    jaguar Well-Known Member

    I re-analyzed the graph with more precision and I came up with these psi amounts for each said CCR:
    1.1 = 1.5 psi
    1.2 = 3.0
    1.3 = 4.5
    1.4 = 6.0
    1.5 = 7.5
    So my upping the CCR by reducing the compressed volume from 200ml to 114ml ups the CCR on my 48cc from 1.1 to 1.2, and on my 55cc reed valved engine from 1.2 to 1.4 which is basically doubling the pressure released into the cylinder when the descending piston uncovers the transfer ports.
     
    Last edited: Dec 6, 2011
  6. jaguar

    jaguar Well-Known Member

    I have completed "stuffing" the crank to reduce the volume by around 86ml. All the JBWeld is drying now and I will test the engine Thursday or Friday. A good part of the volume was stuffed by filling the space to the left and right of the flywheel, at the cases, the 57mm diameter center space. I reduced the crank volume by 20ml on the left side and 40ml on the right side. Inner plates as well as stuffing the transfers and flywheel balancing holes caused the final 86ml reduction. Of course I need to pour in gasoline and measure it to confirm how much difference I made.
     
    Last edited: Dec 7, 2011
  7. jaguar

    jaguar Well-Known Member

    Pouring in alcohol I found out that the final updated CCR is 1.3 for my 55cc reed valved engine. I probably miscalculated how much volume the JBWeld was displacing. So instead of doubling the transfer pressure I only increased it by 50%. Well, that turns out to be a good sized increase to test anyways. So how did it all turn out? First let me interrupt the final announcement to say that these engines have a serious weakness in their crank seals. Increasing the crank pressure did not give me the boost it should have but instead it is acting like the seals are leaking. That is exactly what happened when I first started using a reed valve. So now I have to split the cases again and replace the seals. If the final power boost isn't significant then I'll probably try to remove most of the JBWeld I put on to return the CCR to how it was. There's no need to make my ride unreliable.
    If anyone knows where to buy crank bearings that have a seal on them then please let me know. The ones I have on now do not which puts all the stress on the seals located to the outside of the bearings.
     
  8. a/c man

    a/c man Member

    Last edited: Dec 10, 2011
  9. cloud_2901

    cloud_2901 Member

    I just went to my local bearing shop, he had proper NSK sealed 6202's, picked up heaps of em, $3 each.
     
  10. jaguar

    jaguar Well-Known Member

    Thanks for the help with the bearings.
    I replaced the seals w/o having to split the cases by just prying them out and pressing in new ones. The right seal was worn out but the left seal still had some life left.
    After increasing the jet size (making it richer) there was around a 5% speed boost going up hill at 28mph instead of 26 as before. But lower speeds and top speed is less. I need to experiment with the transfers shape/height and the boost to match the new compression ratio.
     
  11. jaguar

    jaguar Well-Known Member

    I have done some porting experiments w/o success. Could be that the CCR needs to be in relation to both the transfer roof angle and the engine size. Bigger engines mean there is more distance for the intake charge to travel from the transfers to the spark plug area. So then a higher CCR is more befitting a large engine than a small one.
    If I can´t get the transfer roofs flat then I will remove most of the JBWeld to bring back down the CCR. It´s hard to work on the transfer roofs with these small engines.
     
  12. jaguar

    jaguar Well-Known Member

    Here's a section from a web site about this topic:

    Primary compression

    Modern short stroke water cooled engines that employ large transfer port areas do not require a high primary compression ratio, in fact a high primary compression ratio substantially increases the pumping losses and decreases the power available. However, this does not apply to the long stroke of the Villiers unit that uses small transfer ports, even though pumping losses do increase there is a net increase in power. For road racing a minimum geometric primary compression ratio of 1.4:1 is required, 1.5:1 would be better but it is difficult to achieve without extreme measures. The higher ratio is used to squirt the charge through the narrow port. The speed of the charge entering the cylinder is further enhanced by the narrowing of the port, its inlet area should be at least 150% larger than its outlet. A good primary compression will improve tractability and performance whether for road or off-road riding, don't allow this pressure to be diluted through the use of a hollow gudgeon [piston] pin.
    Modern engines should be considered as having transfers fulfilling a "store and forward" function for the fuel with the real control being exercised by the exhaust. The modern exhaust shape required easy access to a supply of fuel mixture, exactly what the large transfer system supplies. What a modern system cannot do is sustain a long deep draw of fuel from the crankcase through inadequate transfer passages.

    In the long stroke small transfer engines, there is significant advantage to be gained from the truly explosive entry of the compressed gases from the crankcase into the cylinder, sweeping the remaining exhaust gases out into the exhaust port in a manner described by Schnurle many years ago. As the piston descends and before the transfer ports are uncovered, a hollow gudgeon pin will bleed off some of the primary pressure that you have fought to create. The hollow pins should be plugged with an alloy slug or slugs which should have no more than a 1 thou clearance fit and be coated with Loctite or similar. Interference fits will swell the gudgeon pin causing fitment problems. The lack of interference fit is required to allow the Locktite to key and bond, a zero or interference fit will not allow the Locktite to work properly. At racing speeds, the difference in power can be measured on the dynamometer.

    The geometric primary compression ratio (GPCR) is useful in determining whether crankcase stuffers are necessary. Be under no illusion: good primary compression is important. If your GPCR is less than 1.2 then action is definitely required.
     
  13. jaguar

    jaguar Well-Known Member

    I just found an important part of this puzzle; fuel atomization. When the intake charge enters the crankcase it is exposed to a lot of hot surfaces that heat the air/fuel up to around 250 degrees (see http://www.charlesriverrc.org/articles/flying/michaelkuper_2strokewarmup.htm) which atomizes a lot of the fuel and makes it where it can burn faster which contributes to a stronger downward force on the piston.
    I used JBWeld which is somewhat insulative. It is on the insides of the flywheels which is a large surface area. I am going to take it off and leave the large mass of JBWeld in place on the cases near the crankshaft.
     
  14. jaguar

    jaguar Well-Known Member

    Here is the before and after results with increased ratio. 48PP is my 48cc piston port top end. 55RV is my 55cc reed valved top end.
    ______5* street__flat street
    48PP__19.5mph__30mph (before)
    ______24.0mph__32mph (after)
    55RV__27.5mph__33.6mph (before)
    ______26.0mph__32.5mph (after)
     
  15. jaguar

    jaguar Well-Known Member

    I just moved down from 8500 feet altitude to sea level and the engine loves it and easily reached the peak 34.5mph that it was modified for. But this place lacks hills to test on and so an essential element I need for testing is missing. I don't really need the bike here so I may take it back up to the mountains where I will be twice a month. Then I can see what effect flattening the transfer roof and/or lessening the crankcase ratio has.
     
  16. lazylightning@mail.r

    lazylightning@mail.r Active Member

    Hi!

    I recently read about some refference to jbweld on an overclockers forum. The man used jbweld to glue a cooling block to a processor chip on his video card. He swears that it is cooling better now than with the normal heat transfer paste. Thus he stated that the jbweld is an effective heat transfer material and not having a heat insulative effect. At any rate, if one really wanted to find out, then overclocking a chip and monitoring it's temperature before and after using jbweld as a heat transfer material between chip and cooling block would probably be the best way find out for sure. At any rate, it seems he may be correct since these overclocker guys are really interested in the best cooling effects and always make something interesting for that. I even remember seeing someone try to make a system with liquid nitrogen.
    Maybe it's of no use to you, since the higher crankcase ratio has harmed the crankcase seals anyway and you should remove some of the jbweld. Just thought it would be interesting for you to know.
     
  17. jaguar

    jaguar Well-Known Member

    thanks for the info.
    I have decided to leave the JBWeld as-is for the time being.
    I flattened the transfers roofs a bit and got more top end speed.
    But unless I take it back up into the mountins I wont know about how it now climbs.
     
  18. lazylightning@mail.r

    lazylightning@mail.r Active Member

    Maybe someday I will go as far as to try to stuff the crankcase with jb weld. But I'm afraid to bother with removing the engine and opening it up now. I already shaved the head down and polished the combuster hemi-chamber in the head and it got alot more power from it. Now I'm driving in the snow and I dont have my winter tires ready yet. So I'm afraid to really even use the power. When I wind it to WOT, I still get the feeling that it could use more gas at the top speed, I feel it starve a bit. I've already drilled out the jet to .8 mm and I use the most lean needle position to get a faster low end response. Otherwise it just bogs down from too rich a mix when I try to accelerate from a low speed. Maybe I could go with adding solder to widen the needle to limit gas flow at low throttle and have more gas flow at WOT with the jet being drilled out a bit more. Winter air is cold though, and it may be all different in the summer. Dreaming of studded tires, expansion chamber and at last the modified cdi. So, the crankcase will have to wait =--)))
     
  19. Ivan H

    Ivan H New Member

    I used sealed NTN brand 6202 bearings. I removed the seals & washed out the lubricant then replaced 1 seal on each bearing & installed with the seals to the outside. I also used them on the countershaft as are. If having trouble finding they're available from Rock Solid Engines in Australia, tho I got from bearing suppliers. They're a quality bearing. Cheers
     
  20. Ivan H

    Ivan H New Member

    Sorry guys, that post was in answer to this, asking about sealed crank bearings. Cheers
     
Loading...