rex@eneevax.UUCP (Rex Root) (03/19/85)
I'm just wondering ... is the sensor looking at the crankshaft or is it looking at the camshaft ... seems to me that the camshaft would be more appropriate since it's spinning at half the speed as the crankshaft. A few manufactures are now running the distributor directly off of the camshaft.
phil@osiris.UUCP (Philip Kos) (03/20/85)
> I'm just wondering ... is the sensor looking at the crankshaft > or is it looking at the camshaft ... seems to me that the > camshaft would be more appropriate since it's spinning at > half the speed as the crankshaft. A few manufactures are now > running the distributor directly off of the camshaft. The sensor could look at either. Just because the crank is spinning twice as fast as the cam doesn't mean it won't work - you just put two lobes on the distributor point cam instead of four (or however many cylinders your engine has). As for running the distributor off the cam instead of the crank, this has been the way it's done for years, because the distributor classically turns at the same speed (in rpm) as the cam, which is half crankshaft speed in a four-stroke engine (1:1 gearing). The same distributor could be driven off the crank with a 2:1 gearing reduction, or a different distributor (with half as many lobes on the point cam) at 1:1. Sounds like Saab is probably using the 1:1 crank drive with two lobes instead of four. Of course, for 2-stroke engines, all this changes. Phil Kos The Johns Hopkins Hospital
tron@fluke.UUCP (Peter Barbee) (03/23/85)
In article <264@eneevax.UUCP> rex@eneevax.UUCP (Rex Root) writes: >A few manufactures are now >running the distributor directly off of the camshaft. This is hardly new (as implied by ...are now...), Bugatti did this in 1935 and he was probably not the first. Also, someone recently mentioned the Toyota Coralla GT-S (?), calling it a sports car. Obviously this car is more 'fun' to drive than your typical Plymouth but sports car? Give me a break. Peter Barbee
prins@cornell.UUCP (Jan Prins) (03/23/85)
>> I'm just wondering ... is the sensor looking at the crankshaft >> or is it looking at the camshaft ... In article <189@osiris.UUCP> phil@osiris.UUCP (Philip Kos) responds: >The sensor could look at either. Just because the crank is spinning >twice as fast as the cam doesn't mean it won't work - you just put >two lobes on the distributor point cam instead of four (or however >many cylinders your engine has). ..... Sounds like Saab is probably >using the 1:1 crank drive with two lobes instead of four. You seem to be laboring under the assumption that a distributor shaft is present: the 'sensors' referred to actually replace the cam that actuates the ignition points and turns the distribution rotor. As for spark timing, you are right, this can be derived from crank or cam. But distribution of the spark to the right plug takes more care. Don't forget that a complete engine cycle requires two crank revolutions. Crank position doesn't tell you what 'phase' of the cyle you are in. The ambiguity requires you to fire 2 plugs at the same time, one of which will be wasted at best, but at worst could cause intake fires with long duration cams. In any case, spark plug life is cut in half and extra energy is required from the coils. Nevertheless, the fully electronic ignition systems I know use crank position and accept this loss because it reduces by half the number of coils required. In Saab's case, they have sprung for the full complement of coils, so there is no excuse for not 'doing it right'. Thus they should take engine position from the cam, or disambiguate crank position by some cam reference. (It may be that the cam position is too "sloppy" because of the chain drive to use for Saab's high-accuracy spark timing program, but I really doubt it). jan prins {vax135, decvax}!cornell!prins