What determines the redline of an engine?

[toph99]

is it just the block/head strength and the strength of the bolts? or are there other factors?
thanks

 

[Amused]

When a piston flies out of the hood.

 

[The Stigenator]

I thougtht it was power to weight ratio in terms of RPM.

 

[Viperoni]

Usually it's valve float, but it could also be the sheer rotating mass of the engine components.

 

[toph99]

so how would someone raise the redline of an engine(like how does the S2000 engine have a 9000rpm redline, and whatever 10's of thousands the F1 cars run at)?

 

[blueghost75]

excelent question

 

[Tominator]

The maximum RPM at which the maximum horsepower can be produced without adverse wear.

My 900 Kawasaki bored to 1150 would destroy engine cases with regularity! MY RD400 Yamaha would turn 12000 revs but quit making HP at 10500...

It's a design limitation.

 

[toph99]

Tominator,
excuse my ignorance but are those motorcycle engines two stroke or 4 stroke? i always noticed that 2 stroke rev higher than a 4 stroke, which is what got me thinking that it had something else besides the actual strength of the block and heads that made the red line.

 

[halik]

well this leads to another question, what determines the powercurve?

 

[blueghost75]

i notice that my truck, which has an 8 cyl 5.7 liter engine redlines at 4000 RPMs, and stays at very low RPMs in normal driving, but the other two 6 cyl engines that I have driven redline at above 6000.

But to contradict that, I rode in a 540i beamer the other day, and it has an 8 cyl engine, and redlines at 6000-7000 RPMs.

 

[toph99]

also, why do diesles have a much narrower power curve and a lower redline? it's the same as a gasoline engine, just different fuel and no spark plugs(gross generalization, i know) but still has the same basic parts, cams, oil pump, crank, valvs etc. so why the low redline?

 

[Tominator]

Both were 2 strokes.

It has a lot to do with rotateing mass and balance...and harmonics!

There is no valve train with a 2 stroke so in theory it should be able to rev higher due to less mass.

Honda attacked this principle in the late 70s. At that time most engine failures in 4 strokes were related to pistons and bearings. They had a motor with pistons that canted in their cylinders and eliminated the wrist pin! Yes, the cylinder was shaped like an hourglass!! At the time it was an unbelievable design. It never matured though...

It eliminated a very weak part of the engine.

Remember at the time metalurgy was in it's infacy and 4 valve heads were only a dream! The sections between the valves would get so hot they would literally become fluid and destroy the motor!

Today the manfacturers tolerences are very close and metalurgy is a science. With less movement and fewer parts in the rortateing mass, tolerences are very strict. That allows for parts that are pre stressed and a minimum of mass.

Combine that with electronic fuel metering and you get higher revolutions....

BTW, the autoparts store I managed sponsered a car that got 10000 rpm from a small block chevy on the track! Professional racrs get 11000 from some pushrod engines! Rebuilds were frequent! LOL!

 

[MrCraphead]

Shouldn't this go in highly technical? all this stuff is going over my head

 

[toph99]

what exactly is rotating mass? i have a general idea(rotating parts) but i don't see what it does/how it effects things

thanks for the info tominator

 

[TrevorK]

I think it would have something to do with the valve float, as mentioned before. You'll notice that smaller engines(Lower displacement) USUALLY rev higher because they need that to generate power, such as Honda's/Toyota's/etc...

You can "change" the redline by removing a rev limiter from your car (Most newer ones have this). Of course, you'll need to upgrade the valves/springs if you don't want things shooting through your hood


Trevor

 

[Mr.IncrediblyBored]

<< what exactly is rotating mass? >>


As the RPM's go up, the speed of the pistons, crank, valves, etc increases as well. When the speed of the pistons increases it creates more momentum and thus more force on the crankshaft (I believe the piston assembly in the cylinders are the "rotating mass"). All this movement puts a lot of stress on the inner components of the engine. Things start overheating and breaking. The redline is simply the point before the engine blows up.

 

[Emulex]

most redline's are about 1000 too low on consumer cars. Although the car may not make much more power up high, a chip can come in handy to extend your powerband..

My car originally had a 6200 redline, i programmed the chip to 7200.. Much better.

 

[Mr.IncrediblyBored]

What kinda car? I bet the engine screams in agony at that high a rev.

 

[Tominator]

..to add...

Today's engines are very balanced due to better manfacturing than a few years ago. All the componets are made at closer tolerances. This allows less clearance in the assembly process to be maintained. I other words, less 'breakin' is needed.

Most valvetrains in modern engines are well up to the task of a high rpm motor. Valve float is not even discussed in modern engines. Before a motor is ever made to get a specific rpm the head design is the very first consideration as it is the only limiting factor.

Yes, it is computer modeling that makes such designs possible! Stress and metal fatigue. Heat and it's effect on components can be explored without actually making a motor!

 

[CocaCola5]

How reliable do you want it? You can probably rev a caddy to 8000rpm but it won't last very long. Same with F1 engines running at 19,000rpm, they don't last for much more than 2 hours of use, just long enough to finish a race. But as revs go up the cost to make the engine go up also, you have to use better materials for all parts in the engine.

 

[Zenmervolt]

Redline is mainly limited by valvetrain ability, namely the ability of the valves to slam shut fast enough that the engine can retain compression. Also, a large bore/short strok engine will have a higher redline because the pistons have less distance to travel with each time the crankshaft revolves, which allows the mean piston speed to be lower at a given speed. With long stroke comes a higher mean piston speed, and for a higher mean piston speed, the connecting rods/pistons need to be more solidly attached. Basically higher engine speeds require more durable componants and these componants are costly. Also, the majority of people need engines with power in lower areas of the RPM band because of the types of driving normally done by a consumer. Revving an engine to the sky may be great for the track, but it can become tiring while driving around the city.

Zenmervolt

EDIT: I don't know for sure, but I suspect that the reason for the lower red line in a Diesel engine is because of the much higher compression ratio of a Diesel engine. A Diesel must work much harder to compress the air inside the cylinder in order to heat it up enough for it to spontaneously ignite the atomised fuel that is injected. Also, most Deisels are designed for maximum low end torque, and in such designs the stroke of the engine is typically very long, and a long stroke which also lowers redline in general. Basically Diesel engines have different design parameters than normal engines and as such a comparison between the two is not really fair.

 

[Tominator]

In diesels we are back to rotateing mass. However, since they work with torque instead of horsepower it doesn't matter much. Modern diesels make usuable power in the 1200 to 2100 rpm range.

 

[Fandu]

Interesting discussion guys. But all this talk brings up another question. Why such a relativly low redline on the Mazda Wankel engines? I mean there are no valves to float, no crankshaft (well very small anyways) to vibrate, etc, etc. Ideas?

 

[Zenmervolt]

<<Why such a relativly low redline on the Mazda Wankel engines? I mean there are no valves to float, no crankshaft (well very small anyways) to vibrate, etc, etc. Ideas?>>

Well, the eccentric shaft (what is the crankshaft in a non-wankel) is not perfectly straight, so there is some vibration, but you seem to have realised that. I believe that it boils down to rotating mass again, and the ability of the apex seals to maintain a good seal against the rotor housing. I'm likely to be wrong, but this seems intuitive to me.

Zenmervolt

 

[Tominator]

Not to appear as a know it all, but why rev higher than a task requires?

The Rotary was not an efficient design to begin with although it is great for a power to weight ratio. Why rev any higher than the operating range? The rotary was a hard motor to get pollution requirements met as well ? Why go higher?