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Efficiency and high compression idea...

BruceMe

Well Known Member
Given this group, this has probably been discussed. But this is an idea that's been pinging around my head for the last few months. I wanted to see what the thoughts of this group were.

This idea came from turbo's running low compression ratios because they have higher effective compression-ration (CR) from the boost. What about the opposite, can you run higher compression and low-enough manifold pressure to avoid detonation and get an effective increase in high-altitude cruise power?

Ideally, optimize the CR for a WOT at 9,000' DA and get an effectively 9:1-like charge density at 9,000' DA. It's 74% of atmosphere at 9k, so 12.2:1, but you would hard-limit manifold pressure to 21.4"

So there is a power reduction at all altitudes (you can't make full-rated manifold pressure). 21.4" takes an 8.7:1 "high compression" O-320 from 160hp (WoT@2700rpm) to 105hp(@2700rpm) on the Lycoming HP chart. But, higher compression does generate more power.

Here's a reasonable SWAG -

7:1 vs 8.7:1 is a 7 hp difference, which means 4.11hp/point-compression-ratio.

I doubt this relationship is linear, but for the sake of an estimate...

4.11 * (12.2-8.7) = 14.4hp + 105hp => 119.4hp at 21.4", but you'd get that power rating all the way up to 9k.

How much efficiency would this gain? So same maths as above, different approach. I would have been making 98hp at 7:1, but using the same amount of fuel, I'm making 119.4hp, or a 21% increase in theoretical spec fuel efficiency... That's a lot, probably too much. But let's say I was burning 8gph, I could be burning 6.6gph to get the same power/same speed.


Is anyone doing anything like this, and what is the drawback?
- Reduced take-off power (see above)
- Additional pilot workload
- Risk for detonation
- Difficult starting

Where am I going wrong? What's wrong with this thinking?
 
High Compression

I have 10:1 pistons in a 0 320 in a non RV. Balanced with carb and Bendix mags. I figure I am getting 168 hp at sea level standard day. The big question I have is whether 10-1 will work if unleaded fuel becomes standard.
 
What about the opposite, can you run higher compression and low-enough manifold pressure to avoid detonation and get an effective increase in high-altitude cruise power?


Is anyone doing anything like this, and what is the drawback?

Where am I going wrong? What's wrong with this thinking?

Your thinking is valid from the engine performance standpoint, although, the compression and firing pressures would be the constant factor you would want to aim for. Also there is a diminishing return on Cr for various reasons given some fixed geometry constants. I have been thinking about 10:1 for this very reason too.

According to Kevin Hortons Cr vs power spreadsheet you are spot on for the power.

Possible issues are piston to head clearances and valve opening clearances to the piston. You do point out that you would have a low altitude limit on MAP to remain within pressure limits. Also, a fully controlled A/F and timing map vs MAP, RPM, and in-man-T would seem prudent to keep pilots out of the equation. (edit - Ross I was thinking about SDS here)

I have 10:1 pistons in a 0 320 in a non RV. Balanced with carb and Bendix mags. I figure I am getting 168 hp at sea level standard day. The big question I have is whether 10-1 will work if unleaded fuel becomes standard.

Jim - I have long been concerned about the detonation limits of unleaded until I watched the (George Braley) GAMI interview on the fuel. It was stated that G.Braley reran the GUL100 detonation test and did not find an overboost limit that triggered detonation. The link was posted here a while back and was over an hour long. If this is true, then the FAA, and EPA should be providing free fuel for that event just to generate some good press.
 
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Dave Anders ran a 12 to 1 AV 360 a few years back and it worked really well at altitude.

Quite workable with variable ignition timing (MAP referenced), you could limit timing just short of detonation.
 
I've put some thought into it too and came to a similar conclusion, albeit you did more math than I did. However, I noted one big drawback and you did as well:

Additional pilot workload

Take-off or go-around being the big high workload times. We want our heads outside, keeping everything straight and upright. Shouldn't be looking inside fixating on 21.4" to avoid detonation.

A hairy go-around after some deer run in front of you in the flare. You push it up to 26" by accident, start detonating and damage the engine. Pretty quickly you've lost any savings from the slight increase in fuel efficiency gained by the high compression ratio.
 
I've put some thought into it too and came to a similar conclusion, albeit you did more math than I did. However, I noted one big drawback and you did as well:



Take-off or go-around being the big high workload times. We want our heads outside, keeping everything straight and upright. Shouldn't be looking inside fixating on 21.4" to avoid detonation.

A hairy go-around after some deer run in front of you in the flare. You push it up to 26" by accident, start detonating and damage the engine. Pretty quickly you've lost any savings from the slight increase in fuel efficiency gained by the high compression ratio.

Way too easy to do. I've had a few occasions where I jammed the throttle in for one reason or another while still LOP and advanced ignition but low enough altitude so the MAP could do some damage, and I'll happily put myself in the top 10% of engine management people. Given the opportunity and capability, the old insurance saying of "Those who have, and those who will" is certainly true.

A true engine management system like SDS can, if properly mapped, immediately move your engine back into a happy place when you do that - but you must have it properly mapped.
 
Dave Anders ran a 12 to 1 AV 360 a few years back and it worked really well at altitude.

Quite workable with variable ignition timing (MAP referenced), you could limit timing just short of detonation.

I talked with Dave for a good 6hrs at OSH17. Took tons of pictures of his 4, I tried to build my 4 similarly, but I fell far short of his. He had so many little (and a few big) things that where tight, it was _ALL_ about detail. Given what I know of him, he probably pulled back from the 12:1 because it was too aggressive and didn't run as he'd like for just flying his wife cross country. He probably didn't want to have to worry about a cracked cylinder head far from home.
 
Take-off or go-around being the big high workload times. We want our heads outside, keeping everything straight and upright. Shouldn't be looking inside fixating on 21.4" to avoid detonation.

I'm thinking I'd put a detente in the throttle, make it easy. Also I don't think 21.4 is some magic number where detonation will occur... I'd want to instrument (EIS + knock detector), fly and boroscope, repeat, but even full sea-level pressure for 30 seconds might not cause damage.
 
I talked with Dave for a good 6hrs at OSH17. Took tons of pictures of his 4, I tried to build my 4 similarly, but I fell far short of his. He had so many little (and a few big) things that where tight, it was _ALL_ about detail. Given what I know of him, he probably pulled back from the 12:1 because it was too aggressive and didn't run as he'd like for just flying his wife cross country. He probably didn't want to have to worry about a cracked cylinder head far from home.

The drop from 12 to 10 lost some efficiency which Dave was troubled with but he added the EFI and started experiments with induction tuning to try to make that up.

Dedicated race and record attempt aircraft can narrow their compromises for better performance of course and Dave mainly uses the plane for day to day ops now.

With user programmable timing, the system takes care of things for you so not much risk there. I wouldn't even attempt this with mags or non-programmable ignition timing.
 
Compression

10-1 pistons in either the 320 or 360 parallel valve-full throttle at sea level standard day is not a problem 10-1 is quite common in aerobatic community and those airplanes are often run full throttle as high as 3500 rpm with fixed pitch prop.
 
I'm thinking I'd put a detente in the throttle, make it easy. Also I don't think 21.4 is some magic number where detonation will occur... I'd want to instrument (EIS + knock detector), fly and boroscope, repeat, but even full sea-level pressure for 30 seconds might not cause damage.

That detent will have different MAP at different altitudes and even altimeter setting. 29.40 is low and 30.40 is quite high, but that's 1"MAP difference right there. Last summer, I saw 28.95 altimeter setting. Airport elevation will have an even greater effect on where that throttle position detent puts the manifold pressure. Every 1000msl difference in airport elevation will change the detent's MAP by 1". My point is a "detent" really only works with FADEC engines, where a computer senses the engine parameters and adjusts the throttle.

I like the idea of high compression and limiting MAP. It's been done on larger engines with co-pilots and flight engineers. But is unnecessary complexity on on our airplanes.
 
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