Hi, looking for some advice for a group project I'm working on for Uni. What we are doing is building a car for the Shell Eco Marathon. However, instead of building the car from scratch, we are building upon and improving work which last year's Masters students done. They've already got a basic car going; they built the chassis, bought and mounted an engine and mounted a steering system. Our goals for this year is to tweak whats there and also design and build a new body.
My main task is to fix and improve the clutch. One of the students said to us that the clutch is slipping and sometimes does not engage correctly, however I've yet to see it in person. So far we don't really have much information on the clutch in terms of who made it, we just know it had to be specially made for us and sent over from America and that it is a centrifugal clutch. Because we haven't had it on the dyno yet, I don't know if the clutch is slipping because of bad gearing or if the friction surface has already worn down. The main problem we are having is that the engine we have uses a 3/8" diameter crank bore, where has most go-kart clutches are 3/4 or 1", so finding off the shelf replacement parts in the UK is pretty tricky. Another point to add is that we've converted the engine to run on bio-ethanol, so I'm not sure how much difference this new heating value will change the power/torque curves
Another problem here is our goal for the car. We are competing in the Shell Eco Marathon and we are not being judged on speed, we simply need to cover the most distance out of 1 litre of fuel, does not matter how fast we go. So my main concern is that centrifugal clutches are happy under full engine load, which for this engine is 7200rpm, which isn't exactly fuel efficient. Ideally, I want the engine to run at around 4400rpm where peak torque is, and maximum fuel efficiency will be (power curve at end of post) . We are also wanting to implement a stop start switch, allowing us to coast as much as we can on the track, however I've read that is bad for a centrifugal clutch and could cause it to burn out?
An alternative I've been looking at is what seems to be called a Torque converter, which confuses me since it is a CVT, not a torque converter.
https://www.gokartsupply.com/tavapp.htm
My main concern is that they mention about having adequate clearance to fit it on the engine/chassis so I'm not sure if this will be viable for our project? (pictures bottom of post) Also, it does cost around 200 quid which is pretty expensive if it doesn't turn out to work for us. Does anyone know if there are 3/8" shaft versions of these?
Again, another thing I'm concerned about is if our engine can produce enough torque at lower revs to be able to pull off without stalling. The clutch we have engages at 2,200rpm, but last year's student said something about having a 2:1 reduction ratio from the engine to the clutch, I'm guessing so the engine can run at 4400rpm for peak torque while engaging, but I can't see where the reduction is happening.
My other option I'm guessing then is to just buy new springs with a lower stiffness constant so they will engage the friction material at a lower rpm? But that won't solve the problem of the engine running at 7200rpm instead of 4000rpm which will be a lot more efficient.
The clutch has a 12 tooth sprocket, which is chained directly to a 60 or 72 tooth sprocket on the driven axle which I believe is 24 inches, although we will be changing that rear wheel. The photos I have posted show the engine connected to the dyno, so ignore what the engine is currently connected to.
My main task is to fix and improve the clutch. One of the students said to us that the clutch is slipping and sometimes does not engage correctly, however I've yet to see it in person. So far we don't really have much information on the clutch in terms of who made it, we just know it had to be specially made for us and sent over from America and that it is a centrifugal clutch. Because we haven't had it on the dyno yet, I don't know if the clutch is slipping because of bad gearing or if the friction surface has already worn down. The main problem we are having is that the engine we have uses a 3/8" diameter crank bore, where has most go-kart clutches are 3/4 or 1", so finding off the shelf replacement parts in the UK is pretty tricky. Another point to add is that we've converted the engine to run on bio-ethanol, so I'm not sure how much difference this new heating value will change the power/torque curves
Another problem here is our goal for the car. We are competing in the Shell Eco Marathon and we are not being judged on speed, we simply need to cover the most distance out of 1 litre of fuel, does not matter how fast we go. So my main concern is that centrifugal clutches are happy under full engine load, which for this engine is 7200rpm, which isn't exactly fuel efficient. Ideally, I want the engine to run at around 4400rpm where peak torque is, and maximum fuel efficiency will be (power curve at end of post) . We are also wanting to implement a stop start switch, allowing us to coast as much as we can on the track, however I've read that is bad for a centrifugal clutch and could cause it to burn out?
An alternative I've been looking at is what seems to be called a Torque converter, which confuses me since it is a CVT, not a torque converter.
https://www.gokartsupply.com/tavapp.htm
My main concern is that they mention about having adequate clearance to fit it on the engine/chassis so I'm not sure if this will be viable for our project? (pictures bottom of post) Also, it does cost around 200 quid which is pretty expensive if it doesn't turn out to work for us. Does anyone know if there are 3/8" shaft versions of these?
Again, another thing I'm concerned about is if our engine can produce enough torque at lower revs to be able to pull off without stalling. The clutch we have engages at 2,200rpm, but last year's student said something about having a 2:1 reduction ratio from the engine to the clutch, I'm guessing so the engine can run at 4400rpm for peak torque while engaging, but I can't see where the reduction is happening.
My other option I'm guessing then is to just buy new springs with a lower stiffness constant so they will engage the friction material at a lower rpm? But that won't solve the problem of the engine running at 7200rpm instead of 4000rpm which will be a lot more efficient.
The clutch has a 12 tooth sprocket, which is chained directly to a 60 or 72 tooth sprocket on the driven axle which I believe is 24 inches, although we will be changing that rear wheel. The photos I have posted show the engine connected to the dyno, so ignore what the engine is currently connected to.
