We've had some people on the forum ask why we feel it's so beneficial to have user adjustable ignition timing. First thing to say is, maybe it isn't so important if you fly at medium to lower altitudes ROP. However, if you fly high and/or LOP we can see substantial benefits in advancing ignition timing under those conditions. The charts below may help you understand why:
The chart above shows how flame speed within the combustion chamber varies with air/ fuel ratio (AFR). Note how much slower the flame speed is at lean mixtures. This means with fixed timing, peak cylinder pressure (PCP) will arrive too late when running LOP to get the most push on the piston, rod and crank for the fuel burn.
The chart above shows the relationship between AFR, power and BSFC. Peak EGT occurs around .065, best power at around .078. Note how power drops off rapidly on the lean side of peak but also how BSFC is best at around .055.
The chart above shows flame speed vs. manifold pressure. Again you can see at lower MAP, flame speed drops so we'd have to initiate the spark sooner to optimize the point of PCP. This again shows the compromise of fixed timing like mags have and why EIs give more speed and/or better economy in cruise, especially at higher altitudes where the MAP falls off.
If we can get all cylinders to peak nearly simultaneously and still light off the lean mixture at the right time to achieve PCP at the optimal point we can maximize the amount of energy we extract from each pound of fuel. Witness Dave Anders' RV4. 200 mph TAS on only 6.8 GPH. Granted, this is a clean aircraft compared to other RVs but if you work out the power this engine is likely making at the MAP and RPM he's running, the BSFC is probably better than .35 lbs/hp/hr.
Is this possible? We know the Wright R3350 turbo compound engines were around .375 running LOP. They were recovering energy back into crankshaft through 3 Power Recovery Turbines but at the same time, they had much lower compression ratios than what Dave is running ( 6.7 vs. around 10 for Dave presently) The higher CR boosts thermal efficiency substantially. We also know that the Continental 550 engines can achieve something around .38 with relatively low CR, fixed mag timing and mechanical injection.
It's not much of a stretch to see that with the higher CR, superior EFI fuel atomization and equal mixture distribution along with variable and optimized ignition timing, we can do better than the the older technology.
Toyota recently achieved an amazing 40% thermal efficiency on their spark ignition 2016 Prius engine. It uses similar thoughts- high CR, lean cruise mixture and advanced timing along with careful EGR feed.
It would be very interesting to put a strain gauge on Dave's crank to measure true torque and therefore hp so we could calculate his BSFC accurately.
Thanks to Liston for the charts.
The chart above shows how flame speed within the combustion chamber varies with air/ fuel ratio (AFR). Note how much slower the flame speed is at lean mixtures. This means with fixed timing, peak cylinder pressure (PCP) will arrive too late when running LOP to get the most push on the piston, rod and crank for the fuel burn.
The chart above shows the relationship between AFR, power and BSFC. Peak EGT occurs around .065, best power at around .078. Note how power drops off rapidly on the lean side of peak but also how BSFC is best at around .055.
The chart above shows flame speed vs. manifold pressure. Again you can see at lower MAP, flame speed drops so we'd have to initiate the spark sooner to optimize the point of PCP. This again shows the compromise of fixed timing like mags have and why EIs give more speed and/or better economy in cruise, especially at higher altitudes where the MAP falls off.
If we can get all cylinders to peak nearly simultaneously and still light off the lean mixture at the right time to achieve PCP at the optimal point we can maximize the amount of energy we extract from each pound of fuel. Witness Dave Anders' RV4. 200 mph TAS on only 6.8 GPH. Granted, this is a clean aircraft compared to other RVs but if you work out the power this engine is likely making at the MAP and RPM he's running, the BSFC is probably better than .35 lbs/hp/hr.
Is this possible? We know the Wright R3350 turbo compound engines were around .375 running LOP. They were recovering energy back into crankshaft through 3 Power Recovery Turbines but at the same time, they had much lower compression ratios than what Dave is running ( 6.7 vs. around 10 for Dave presently) The higher CR boosts thermal efficiency substantially. We also know that the Continental 550 engines can achieve something around .38 with relatively low CR, fixed mag timing and mechanical injection.
It's not much of a stretch to see that with the higher CR, superior EFI fuel atomization and equal mixture distribution along with variable and optimized ignition timing, we can do better than the the older technology.
Toyota recently achieved an amazing 40% thermal efficiency on their spark ignition 2016 Prius engine. It uses similar thoughts- high CR, lean cruise mixture and advanced timing along with careful EGR feed.
It would be very interesting to put a strain gauge on Dave's crank to measure true torque and therefore hp so we could calculate his BSFC accurately.
Thanks to Liston for the charts.
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