just a little engine theory question. Assuming equal displacement and cam duration etc. what do the different torque curves of various engine configs look like. I.e. inline 4 cylinder, six cylinder, v-8t, opposed eight, v 12
etc. im just curious.
people always say four cylinders need to be revved to make power, is this because they are usually tiny? or is this an inherent characteristic of this amount of cylinders?

 

Your question has contrdictions built into it. Just using the simple extremes of the V8 and the inline 4, consider this. You ask if the 4 cylinder needs to be revved be because it is tiny. Generally, the answer is yes. They don't have the cubes to produce the torque to do the job, so they are cammed and inducted to rev and make horsepower. But in your opening question, you want the engines to be of equal cubic inches and to be cammed the same. A 350 cid, 4 cylinder, engine would have to have a huge bore and stroke. Genarally it will NOT achieve high revs or great horsepower, certainly not the kind of numbers a associated with a 350 V8. That 350 I4 would probably be good in your tractor, OatBoy. I don't understand the point of the question.

RACE ON!!!

 

well i guess that answers part of my question.

 

The engine shape (straight, V, opposed) has a lot more to do with packaging than anything else. Inline engines are generally stronger though. It would take a lot of room to stuff 8 cylinders in a row under your hood, so that's why there's a V. Oversized Inline engines also have vibration problems most of the time. Its very hard to keep a crank that long from bending too much.

Assuming equal displacement, more cylinders are generally better, but there's a point of diminishing returns. An engine with a big stroke can't be revved high before it rips itself apart (F1 engines have a very very short stroke). If the bore is too big, the fuel and air can't mix as well inside the combustion chamber. This also limits revs to some degree.

 

If you're talking about cylinder count for the SAME DISPLACEMENT, you nearly always want MORE cylinders. Aspiration is limited by mean piston speed, but power is the product of torque and revs, so a short stroke V-12 will outpower any four-cylinder of the same displacement. Peak torque will be in the same ballpark, but the V-12 will make at least three times the peak power.

Cylinder layout is primarily a matter of packaging, and number of cylinders is a matter of cost, packaging, and fuel efficiency. More cylinders usually means higher internal fiction, so idle and low speed fuel efficiency will be worse than an engine with less cylinders.

Look at the EPA fuel economy numbers for a 575 Maranello versus a Corvette. The 575 has very poor fuel economy, partly because it is so heavy, and partly is because the V-12 consumes a lot of fuel, even at normal driving speeds.

Duke

 

OK, I have a corollary engineering/thermo question if anyone has a minute.

On another board someone raised the issue of thermal efficiency for a stroker
- the particular question was framed in terms of BSFC.

In general terms, if you stroke an engine from 3.48" to 3.75" (holding all
else same) would there be a signigicant increase in thermal efficiency / decrease
in BSFC?

Seems like on the positive side the CR & pumping efficiency would go up; but so
would piston friction and heat losses (heat transfer area). There is probably
a net efficiency gain, but I wondered if anyone has a feeling for its magnitude.

I thought it was an interesting topic (but not interesting enough to go break out
a thermo text to calculate an answer :)).

 

It's a matter of the geometry of the combustion chamber, and if you compare a 3.48 and 3.75" stroke the geometries are the same for a given bore and head. One can get into an argument of the combustion chamber shape over the combustion time, and this is essentially the same as the "rod ratio" argument, but within the range of strokes and rod ratios that can be installed in a SB, it is splitting hairs.

An extreme example is current F1 engine combustion chamber geometry. With a bore/stroke ratio of abound 2:1 their combustion chambers are thin disks, which are themally inefficient due to a very high surface area to volume ratio and is one reason why these engines only produce usable power in the upper 20 percent of the rev range, where combustion occurs so fast that there is minimum time for heat transfer to occur.

Duke

 

Duke

Thank you for the info. I could see where a stroker might have slightly better compression efficiency, but nothing to write home about.

Let me belabor the point a bit further - does a solid-lifter cam have a significant effect on engine efficiency? I notice from some dyno charts on the net that some larger SBC build-ups seem to show better BSFCs.

Wondered if that could be attributed to more severe solid lifter cam profile, combined with a higher RPM torque-peak, resulting in more efficient pumping/burning of the air-fuel mix. Just curious.

TIA

 

I'm not sure how accurate the BSFC data are on the dyno programs or what consitutes a "signficant difference". There are a number of factors that affect BSFC - compression ratio, aspiration efficiency, internal engine friction, heat transfer characteristics of the combustion chamber, and air fuel ratio.

A big contributor to thermal efficiency is compression ratio, which can be thought of conversely as the expansion ratio. The more compression ratio, the greater the expansion for a given valve timing, but the primary reason for higher thermal efficiency with higher CRs is the higher average combustion temperature. Basic themodynamics tells us that higher thermal efficiency is obtained by adding heat at higher temps and rejecting it at lower temps.

BSFC is generally best near the torque peak at WOT, and can be very poor at low revs on a highly tuned engine that has high specific output. High rev BSFC typically falls off because engine friction power increases with the cube of speed and eventually causes brake power to fall when aspiration efficiency drops off.

The other limitation of the simulation programs is that they do no show part load BSFC, which are of prime importance to OEMs since they are a big determining factor in fuel consumption.

Duke