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highest reving automobile engine

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86corolla

86corolla

Automotive
Sep 14, 2008
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I'm trying to develope a new head design that wont have any rpm limit. I would like to use an existing long block to lower costs but I'm not sure which would be best. I'm leaning towards a four cylinder HONDA because parts are cheap and available. This would become the prototype engine with a lot of custom work so any engine would actually be fine as long as the long block could survive 10k+ rpm.

Any suggestions
 
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Good rule of thumb, the industrial engines I work with run ~8.5-8.7 m/s continuously for ~4 years without a bottom-end overhaul.

Another is best fuel economy on an automotive engine is ~5-6 m/s.
 
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"Another is best fuel economy on an automotive engine is ~5-6 m/s."

I have never heard of any function that relates piston speed to best fuel economy!?! How on earth does that work then?

MS
 
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um, last I checked the BSFC curve for engines varies with rpm, and cars tend to get better fuel economy in about that rpm range.

regarding direct effects of piston speed vs economy, you do have lower friction losses at lower piston speeds (to a point).

Search for "gear fast run slow" for more info.
Example result: A truck spec'ed for optimum fuel economy will typically be geared to run at a very low engine rpm while at cruise speed, the so-called "Gear Fast Run Slow" concept.
 
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"um, last I checked the BSFC curve for engines varies with rpm"

Come on Mike, I'm sure there is no requirement for sarcasm?

"and cars tend to get better fuel economy in about that rpm range"

A truly nebulous statement if ever I heard one!


That is quite possibly the biggest piece of generalisation I have ever come across - but I guess it was put forwards as a 'rule of thumb'.

MS
 
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Sorry, your questions came across as being either very naive or "bait," either of which justify sarcasm in my book. Go ahead, launch your flame war now.

I'm pretty sure that I've even seen an old engine design reference with a plot of BSFC vs mean piston speed for a large sampling of engines.

Steve, I think that such a graph may be available from a designosaur at your company.

 
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I'm used to Isaac's sarcasm

My question was unrelated to rpm. I was wondering if there was a DIRECT effect of piston speed on economy (a cause) or whether it was indirect (a correlation). If it's purely down to friction, why is there an optimum?

Obviously there are sweet spots on BSFC maps that tend to be in a certain common rpm range. But take a massively over-square engine. Does its BSFC sweet spot move because the rpm/piston speed ratio changes or for other reasons?

- Steve
 
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It's obviously a rough estimate that won't hold true for every automotive engine but that's why it's a rule of thumb. I think it's more of an empirical observation than anything and 5-6 m/s is 5.5 m/s +/- 10%.

Obviously, lower piston speeds means less friction but at some point you start to lose efficiency due to the amount of time available to transfer useful heat out of the combustion gases and into the cylinder, head and piston where it's wasted.
 
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I have trouble with the amount of time argument.

If you do 1000 rpm for 1 minute or 6000 rpm for one minute you still spend 1/4 min on the power stroke, you just have 6 times as many events for 1/6th the time each.

Obviously higher speeds lose more to inertia in the reciprocating parts.

Rate of heat build up vs piston speed reduces the useful work that can be extracted as speed increases.

Rate of burn increases as turbulence increases from quench effect as rpm increases.

Higher rpm allows the same power from a smaller engine which weighs less and may fit in a lighter more aerodynamic chassis.


Regards
Pat
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regardless of the reason for it, it is apparent from a BSFC plot that there is a minimum value which occurs well above idle rpm, and well below maximum rpm. Friction continues to drop as speed is reduced from the peak BSFC point, so that's not the reason for the increase in fuel consumption.

here's a bit of engineering course material from GA Tech which suggests that heat lost to the cylinder wall drops with increased engine speed:

I expect that Steve (Somptingguy) could get us a very authoritative answer from another member of his organization... but it might take more effort than it's worth.

Anyway, someone who's familiar with Woschni's heat transfer work might be able to shed some light on this. Not me!
 
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Hmm, time for some handwaving...

High stoke ratio (for same displacement and rpm) means:

- Lower side loads (more acute rod angle) - less friction
- Higher contact area (bigger piston) - more friction
- Shorter stroke (Integral of force*displacement lower) - less friction

So I've argued myself round in circles. Unfortunately I don't have access to the Chen Flynn paper at home and I'm not in work 'till Monday.

- Steve
 
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