Rotary engine Vs. Piston engine (1 Viewer)

[Riet]

torque is great for the take off/launch, once you get going/momentum, its hp.

This actually isn't true at all. Acceleration exactly follows the torque curve. Horsepower is just of a way of expressing the engines ability to produce torque in a useful rev range.

 

[Graney]

Rotaries tend to have shit amounts of torque (compared to a piston engine) relative to their horsepower because they are usually about 1/3 the displacement.

Given HP is a function of torque x RPM, and rotaries typically rev higher than a piston engine with a given hp, they obviously must have correspondingly low torque relative to their HP.

To generate a given HP, an engine can either have high torque, or high RPM (or something inbetween I suppose). It's impossible to have both.

Relative to their capacity however, rotaries have heaps of torque. You can have a rotary with high torque easily, it'd just have to be built to a larger capacity. A rotary of a given capacity will generate more torque than a typical piston engine of the same capacity. But even if you were to raise capacity, it'd still rev hard, the HP would shoot up as you raised capacity, and for the HP output, you'd still have a relatively low torque to HP ratio.

 

[Graney]

In conclusion, orbital direct injection 2-stroke > all

 

[Riet]

Given HP is a function of torque x RPM, and rotaries typically rev higher than a piston engine with a given hp, they obviously must have correspondingly low torque relative to their HP.

To generate a given HP, an engine can either have high torque, or high RPM (or something inbetween I suppose). It's impossible to have both.

Relative to their capacity however, rotaries have heaps of torque. You can have a rotary with high torque easily, it'd just have to be built to a larger capacity. A rotary of a given capacity will generate more torque than a typical piston engine of the same capacity. But even if you were to raise capacity, it'd still rev hard, the HP would shoot up as you raised capacity, and for the HP output, you'd still have a relatively low torque to HP ratio.

Nah man bigger = less revs, always. To increase capacity mazda have always just used more rotars. Btw, I'm not sure if the horsepower/torque function is the same for rotaries since they don't have the same cycle. But yeah, other than that you're spot on.

(Very) generally speaking most modern naturally aspirated engines get about 100 Nm per litre in sports car applications. Rotaries are definitely better than this, but the torque curve is just as important as the peak, and as rotaries don't have cams (or valves at all, last time I checked) they usually can't as have a good a torque curve. Since they are tuned/timed to produce more torque and thereby power at high RPMs their efficiency over the rev range suffers. I'm also not sure if ignition timing or even air-fuel ratios can be tuned for different RPMs like with a piston engine. This is the main reason they suffer. Just like how high-revving sports bikes tend to have jack all torque low down, especially since they were still carburetted until a few years ago.

Contary to what Az said modern turbos have excellent torque curves, this is probably their main advantage over naturally aspirated engines, rather than just pure peak horsepower.

For the record, power is a linear function of fuel consumption. The differences between engines is purely different volumetric efficiency.

 

[staticsiscool]

i dont know why people get all sooky about different engines

you can do the same shit with all of them when they are tuned correctly

 

[Jake22]

yer just buy a v8 and shutup

 

[Graney]

Nah man bigger = less revs, always. To increase capacity mazda have always just used more rotars. Btw, I'm not sure if the horsepower/torque function is the same for rotaries since they don't have the same cycle.

I never said you had to use a single rotor

If you increase stroke (or general capacity in a rotor I guess?) without doing anything else you'll lose revs.

Now I'm wondering if rotary's have an equivalent for Bore and stroke? Generally, a large bore, short stroke engine is used if you want high RPM. I suppose rotors have two dimensions, the width by depth, what would be the relative performance of an engine with large width, vs an engine that was deep? I suspect the width is equivalent to stroke in a piston engine, but idk.

Btw, I'm not sure if the horsepower/torque function is the same for rotaries since they don't have the same cycle.

I'm guessing that because the engine fires three times for each rotation of the crank, there are 3 RPM in single crank rotation?

 

[Riet]

nah 1 crank rotation = 1 rpm that's why 2-strokes make "double" the power of an equal sized 4-stroke. I'm just not sure how it effects the equation.
(btw guys, kW = (Nm*Rpm)/9550).
The thing about width and diameter being bore and stroke equivalents is interesting but honestly I have no idea

 

[Azamakumar]

For comparison purposes between a Wankel Rotary engine and a piston engine, displacement (and thus power output) can more accurately be compared on a displacement per revolution (of the eccentric shaft) basis. This dictates that a two rotor Wankel displacing 654cc per face will have a displacement of 1.3 liters per every rotation of the eccentric shaft(only two total faces, one face per rotor going through a full power stroke) and 2.6 liters after two revolutions (four total faces, two faces per rotor going through a full power stroke). This is directly comparable to a 2.6 liter piston engine with an even number of cylinders in a conventional firing order which will also displace 1.3 liters through its power stroke after one revolution of the crankshaft, and 2.6 liters through its power strokes after two revolutions of the crankshaft. Measuring a Wankel rotary engine in this way more accurately explains its specific output numbers, as the volume of its air fuel mixture put through a complete power stroke per revolution is directly responsible for torque and thus horsepower produced.

I think the displacement of the engine is the volume of the rotors, so closest to bore x stroke would be length x width?




Also re turboes; you still get turbo lag, even in modern turbos. Sure they put out a lot of power once they reach max boost, but that means that the torque curve has a steep incline. I was just pointing out that the difference between low end torque and high end torque in a rotary isn't as large as a piston engine, turbo or not.

 

[Riet]

Man that's reaching boost threshold, not turbo lag.

"Boost threshold

Turbochargers start producing boost only above a certain exhaust mass flow rate (depending on the size of the turbo) which is determined by the engine displacement, rpm, and throttle opening. Without an appropriate exhaust gas flow, they logically cannot force air into the engine. The point at full throttle in which the mass flow in the exhaust is strong enough to force air into the engine is known as the boost threshold rpm. Engineers have, in some cases, been able to reduce the boost threshold rpm to idle speed to allow for instant response. [3] Both Lag and Threshold characteristics can be acquired through the use of a compressor map and a mathematical equation."

 

[Graney]

I think the displacement of the engine is the volume of the rotors, so closest to bore x stroke would be length x width?

Definetly, I'm just wondering what would be the relative characteristics of a large width rotary vs a large depth rotary. I suspect there may be little to no difference in performance characteristics.

I guess the basic principles of a lever apply. I.E. with a large width, you'd have the outer tips of the rotor moving at a relatively high speed, with relatively low load.

Whereas a rotor (capacity being equal) that was relatively deep, with a small width, would have the outer tips of the rotor moving at a relatively low speed, with a correspondingly high load on the rotor.

I'm assuming that, just like there is an absolute limit on piston speeds in a regular internal combustion engine of some x/m second, there is a similar speed limits on rotors, and the engineering as in all things, is necessarily a compromise.

I suspect ports would be more efficient on a large width rotor.

 

[Riet]

IIRC F1 cars have maximum pistons speeds of around 40 m/s. I think the diameter is most similar in effect to stroke, but who knows? I'll check UNSW library next time I'm there to see if there is any literature about them.

 

[Azamakumar]

so wtf is turbo lag? when the engine reaches boost threshold and nothing happens?

 

[staticsiscool]

turbro lag is caused by the time it takes the turbo to spool up cos its fed by exhaust or some shit so when ur idleing not much exhaust is happening so when u press the trottle the engine increases revs and more exhaust is made and then it starts spinning the turbo and then the turbo starts smooshing in more air or someshit

that is my interpretation

 

[Azamakumar]

thats what i thought too

 

[staticsiscool]

hope im right

thats why blowers dont have lag cos they run straight off the motor via a belt or someshit so it doesnt fuck around with waiting for exhuast pressure to build up, but low revs equal low boost etc. i dunno, i just like 2.8l diesels with 63kw of powers

 

[Azamakumar]

yeah i know, then they reach the boost threshold and a wastegate opens up to keep boost constant etc etc.

But apparently this is reaching boost threshold?

 

[staticsiscool]

im lost

i'd like to direct you to the how should i modify my ute thread now

 

[Riet]

yeah i know, then they reach the boost threshold and a wastegate opens up to keep boost constant etc etc.

But apparently this is reaching boost threshold?

boost threshold is for example, opening throttle at 1000 rpm and not having any boost until 3000 rpm.

Turbo lag is opening the throttle at any rpm and there being a delay until the turbo starts producing boost, but if you are above the boost threshold it will only be for a moment.

Analogy time! Turbo lag = putting foot down in automatic and the torque converter taking a second to load up before you accelerate (if yours doesn't lock automatically). Boost threshold = having no torque below 3000 rpm because the peak is tuned to be much higher.

 

[Riet]

After reading the only book about wankel engines that UNSW has I still have no idea what axial depth has on effective RPM or efficiencies. The book seemed much more concerned with apex seals and port timing issues.