The Technical Questions Thread

[hightower99]

No, ht, just by the way it works you can't get rid of the oil consumption.
Can't use scraper rings so oil will ALWAYS be on the bores.

The oil consumption is mainly due to oil suspended in the air which is burnt because they don't use pressurized oil systems (I know that you know this ).

I believe that there are a few 2 stroke engines that do have oil control rings on the pistons. Normally you wouldn't be able to use them because of the wide ports in the cylinder walls, but if you only have an intake/transfer port which reaches all the way around the bottom of the cylinder then you can put in enough supports to be able to use an oil control ring.

[2ndclasscitizen]

The oil consumption is mainly due to oil suspended in the air which is burnt because they don't use pressurized oil systems (I know that you know this ).

I thought the oil consumption was more to do with the oil being mixed in with the petrol.

[Matra et Alpine]

I thought the oil consumption was more to do with the oil being mixed in with the petrol.

You are also correct.
What ht was pointing out was the oil is in the vapour/mist/fuel-air mix whichever you want to call it

It has to do this as it can't rely on scraper rings and sprayed oil on the bores. IF you go to all the effort of adding valves and compromise porting for scraper rings etc etc then you start to wipe out the some of the two-stroke advantages.

[culver]

I think HT is right. The later automotive two strokes, as I recall, didn't mix oil and gas because, like the two stroke diesels, they didn't run the incoming charge into the crank case. Instead they used a blower to pressurize the air entering the cylinder. My understanding is you still need to make sure you don't allow unburnt incoming charge to exit with the exhaust gases. Diesels don't have that issue as the incoming charge is just air. The fuel is added after the exhaust valve is closed. A direct injection gasoline motor would also get around that issue but I would assume that as of '92 many of the emissions and hardware issues related to direct injection of gasoline engines were not solved. Things might be different these days.

Why can't a 2-stoke use scraper rings and sprayed oil systems if you aren't sending incoming air into the crank?

[hightower99]

It has to do this as it can't rely on scraper rings and sprayed oil on the bores. IF you go to all the effort of adding valves and compromise porting for scraper rings etc etc then you start to wipe out the some of the two-stroke advantages.

It isn't hard to add a few simple exhaust valves and for an engine used in cars if you used a rotary valve it wouldn't hurt revability much either. Keep in mind that a 2 stroke exhaust valve doesn't have to deal with some of the things that a 4 stroke exhaust valve has to. Also adding bridges across the intake port to support an oil control ring is not going to degrade the effectiveness of the ports. In fact many current 2 stroke designs incorperate bridges because the intake ports are getting bigger and bigger. Of course you would need to design an oil control ring that drained almost all of the excess oil into the piston instead of "squeeging" it down the cylinder walls like 4 stroke oil control rings do, otherwise it would just scrape the oil off directly into the intake port

[Kitdy]

Ok two questions. First off, view the photo. What is the large silver object at the back end of this Ferrari F40 GTE?

Second off, how is the boost pressure for a supercharger or turbocharger increased or decreased?

[hightower99]

Ok two questions. First off, view the photo. What is the large silver object at the back end of this Ferrari F40 GTE?

Second off, how is the boost pressure for a supercharger or turbocharger increased or decreased?

First: The large silver object is the main muffler for the F40. It is wrapped in some kind of thermal insulation.

Turbocharger boost control: There is normally a wastegate (which is a valve that allows exhaust gas to bypass the turbine of the turbocharger). If the wastegate is closed the turbocharger will produce the most boost but you might end up choking the exhaust or damaging the turbo by making it work under surge conditions. Normally the wastegate opens gradually which maintains a set amount of boost. So keep the wastegate closed you get max boost up to a point where you have to start opening the wastegate. Some variable geometry turbos don't use a waste gate and rely on the geometry of the interface between the turbine housing and the turbine.

Superchargers: Boost is regulated by the pulley ratio, which determines max boost. There may also be a blow-off valve that bleeds pressure off if it exceeds a set limit. To get more boost from a supercharger you need a higher ratio (ie 1:5 instead of 1:3 (engine RPM:supercharger RPM)).

[Kitdy]

First: The large silver object is the main muffler for the F40. It is wrapped in some kind of thermal insulation.

Turbocharger boost control: There is normally a wastegate (which is a valve that allows exhaust gas to bypass the turbine of the turbocharger). If the wastegate is closed the turbocharger will produce the most boost but you might end up choking the exhaust or damaging the turbo by making it work under surge conditions. Normally the wastegate opens gradually which maintains a set amount of boost. So keep the wastegate closed you get max boost up to a point where you have to start opening the wastegate. Some variable geometry turbos don't use a waste gate and rely on the geometry of the interface between the turbine housing and the turbine.

Superchargers: Boost is regulated by the pulley ratio, which determines max boost. There may also be a blow-off valve that bleeds pressure off if it exceeds a set limit. To get more boost from a supercharger you need a higher ratio (ie 1:5 instead of 1:3 (engine RPM:supercharger RPM)).

Ok, for example how did the Porsche 917/30 get a boost of over 30psi, while some cars are much less?

And what is a pulley ratio?

If i get you correctly, you can have a better turbo boost if you build the turbo out of more exotic/stronger/more expensive materials yes?

[hightower99]

Ok, for example how did the Porsche 917/30 get a boost of over 30psi, while some cars are much less?

They use large turbos that are capable of supplying enough air to create 30psi+ boost. I hope you realise that you can't just turn the boost up to 30psi+ on any turbo, max boost is determined by turbo and engine size. Also the max boost that the engine can handle is almost always less then the max possible boost that a turbo can deliver.

And what is a pulley ratio?

Do you know what a pulley is? Pulley ratio is the ratio between the diameters of two different pulleys. For example if you have a 100mm dia. pulley on the crankshaft connected to a 50mm pulley on a supercharger then you have a ratio of 1:2. There can also be gears involved that increase or decrease the overall ratio.

If i get you correctly, you can have a better turbo boost if you build the turbo out of more exotic/stronger/more expensive materials yes?

Well yes. But it isn't quite as simple as that. Normally new/better materials will make a turbocharger more efficient but that doesn't mean that max boost pressure will go up.

[Kitdy]

Next topic: rotaries.

First off, what is the difference between a 2/3/4 rotor rotary engine?

Why aren't rotaries used in more racing series due to their superior hp/L rating (and hp/kg rating - a 13B in the RX8 has 1.32 hp/lb while the Enzo's F140 has only 0.01 hp/lb more at 1.33 hp/lb)?

Why don't we see any large displacement rotary engines - an example beiing the RENESIS being 1.3L and the Mazda 787b's engine being 2.6L and yet still producing over 700 hp?

Could a racing rotary theoretically rev as high as an F1 engine?

If rotaries are so good, why don't we see more of them road cars?

[clutch-monkey]

Why don't we see any large displacement rotary engines -

47L isn't enough?

[fisetdavid26]

47L isn't enough?

Haha, I think it is.

What was the purpose of that monster?

[clutch-monkey]

to consume oil

[fisetdavid26]

Why aren't rotaries used in more racing series due to their superior hp/L rating (and hp/kg rating - a 13B in the RX8 has 1.32 hp/lb while the Enzo's F140 has only 0.01 hp/lb more at 1.33 hp/lb)?

It's quite simple, they're not allowed thanks to rules in several series. They're banned at Le Mans IIRC.

to consume oil

lmao

[fpv_gtho]

Do you know what a pulley is? Pulley ratio is the ratio between the diameters of two different pulleys. For example if you have a 100mm dia. pulley on the crankshaft connected to a 50mm pulley on a supercharger then you have a ratio of 1:2. There can also be gears involved that increase or decrease the overall ratio.

Wouldnt they calculate that from the circumference of the pulley rather than the diameter?