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?
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?