smokyray
Well Known Member
Disclaimer: I know this is only remotely RV related being that a former RV builder/flyer/owner/experimenter typed it!
However comma, being a current Sonerai 2 driver and now VW engine semi-expert (by default) and given that Van built and flew a VW powered RV, The RV5 back in the early 80's and several RV12 builders on this site have hinted at equipping their RV12's with a VW derivative I thought I would share an excellent thread from the Sonerai.net site.
The subject is heat transfer, common to all air cooled engines and more specifically (something often overlooked in the RV world), exhaust tuning...
The Heat is On...
Seems two related issues: Power curve and heat. Components are related to both issues. More power, higher compression, larger bores = more heat as well as higher pulse energy absolutely and in frequency. These engines are delivering much higher power than in an automobile configuration. The components must handle the dynamic stresses. Cast iron is not a very flexible material but aluminum is. Pushing power is probably going to see more stretch, material elongation, at the the two aluminum (or aluminum-magnesium) ends of the cylinder, and I'd think that would be where loosening action would be found. Aluminum and magnesium also have a much higher expansion coefficient than cast iron, about 2-1. A thin-walled cast iron cylinder might deform as a cylinder (yikes!) but would not deform at its ends if properly seated.
The other issue is heat rejection. The low end of the cylinder does not have a heat issue. The head end has the heat issue. Consider that if at the entrance to the exhaust tube, at the exhaust valve, we have a temperature exceeding 1,400, but we have an internal piston temperature approximately that of the oil, being some 1,000 degrees less, we have tremendous heat flow through the upper cylinder and head. This heat has to be continually and efficiently rejected to avoid heat-related damage. Since cylinder design can only change in two basic respects regarding heat dissipation, wall thickness and fin efficiency, most of the heat and related component expansion/contraction issues are probably related to the design and cooling of the head if the oil cooler is properly sized. Across the head, we have very cool air entering on the intake side and very hot exiting on the exhaust side with a temperature difference of maybe 1,400 degrees. Some of the heat flow is into the intake gasses, not an entirely bad thing. But on the exhaust side, we have this very hot gas stream flowing from the upper cylinder, contacting the exhaust valve, and pulsing through the exhaust port and entering the exhaust pipe. So, in my thinking, the issue is how to maximize the efficiency of heat rejection on this exhaust side after the heat is contained in the head and exiting the exhaust port into the exhaust. This has two major components: internal and external.
Internally, I'd want the exhaust side to be as smooth as possible, ported and polished to remove all ridges and fins that can absorb and transfer heat inwardly to the engine. Polishing reduces surface area available for heat absorption. The idea being to get the maximum amount of heat outside of the engine before the head can absorb that heat. Head fins and engine oil dissipate heat that is absorbed by the head and not exited though the exhaust pipe. Probably a good head design would increase the size of the exhaust port to the maximum possible, which would cool the exhaust gasses exiting the cylinder by expansion. At this point, we are still inside of the head with almost all of the exhaust gas heat. The next step would be to move that heat outside of the head and into the exhaust pipe by two methods. The first would be to use the largest exhaust pipe possible with the least restriction. If the exhaust pipe connection to the head, at least the pipe diameter, is larger than the internal head exhaust port diameter, the gasses will expand again and that will have two effects: first being cooling as an expanding gas, and the second being lower exhaust gas pressure after the head as a result of the gasses expanding into the pipe and cooling. This expansion will also have a scavenging effect, tending to pull hot gasses out of the engine. (All of this discussion is for the non-turbo engine). The scavenging function of cooled and expanded gasses is of course limited by the "closed end" of the head nearest the exhaust valve seat. In this valve-seat area we would want maximum material thickness to retard heat flow into head and the engine. Porting & polishing before and after pics: http://www.aircooledtech.com/exhaust_pp/
The next-to-last component is header design. Looking at how most of us have our exhaust routed, I think much of the heat and related expansion/loosening issues are related to exhaust header design. Most of us have no real design. Just bent tubes stuck onto the exhaust port and directed down. I think most of the bends are too sharp and and the pipes are too small to achieve anything but rudimentary heat rejection, which results in unnecessarily high head temperatures. Some seem the same size as a 1600 on an old Beetle putting out 45 HP. Engine cooling would benefit from both large exhaust pipes and 4-1 tuned exhaust that is optimized to scavenge the exhaust. Scavenging sucks waste heat out of the engine. Here is some discussion on header design for racing engines:
http://www.geneberg.com/cat.php?cPath=15_445
http://www.epi-eng.com/piston_engine_technology/exhaust_system_technology.htm
A collector, ditch the springs!: http://vwparts.aircooled.net/Bugpack-Off-Road-Racing-Collector-1-5-8-2050-22-p/2050-22.htm
Lastly, the idea of ducting the cooling air around the heads, cylinders, and the exhaust using baffling needs some major consideration. Most of the ducted engine cooling air needs to first cool the heads with a lesser flow to the cylinders. Most baffling just directs flow to the fins in general without discriminating between the heads and the cylinders. Once that fin cooling ratio is determined and solved to reject the maximum retained engine heat, the exhaust pipes within the cowling need to be cooled for two reasons: to cool the gasses within the pipes to aid in heat extraction from the engine, and secondly to reduce heat within the cowling, which will reduce engine heat load.
By engineering to reduce engine heat absorption by maximizing engine heat rejection, at least some of the issues related to thermal expansion and head failure can be reduced or solved. Also, an engine that operates with less heat strain should last longer and should have less propensity to fail at higher power outputs.
Anyway, this is my 2¢.
Thaddeus
V/R
Smokey
However comma, being a current Sonerai 2 driver and now VW engine semi-expert (by default) and given that Van built and flew a VW powered RV, The RV5 back in the early 80's and several RV12 builders on this site have hinted at equipping their RV12's with a VW derivative I thought I would share an excellent thread from the Sonerai.net site.
The subject is heat transfer, common to all air cooled engines and more specifically (something often overlooked in the RV world), exhaust tuning...
The Heat is On...
Seems two related issues: Power curve and heat. Components are related to both issues. More power, higher compression, larger bores = more heat as well as higher pulse energy absolutely and in frequency. These engines are delivering much higher power than in an automobile configuration. The components must handle the dynamic stresses. Cast iron is not a very flexible material but aluminum is. Pushing power is probably going to see more stretch, material elongation, at the the two aluminum (or aluminum-magnesium) ends of the cylinder, and I'd think that would be where loosening action would be found. Aluminum and magnesium also have a much higher expansion coefficient than cast iron, about 2-1. A thin-walled cast iron cylinder might deform as a cylinder (yikes!) but would not deform at its ends if properly seated.
The other issue is heat rejection. The low end of the cylinder does not have a heat issue. The head end has the heat issue. Consider that if at the entrance to the exhaust tube, at the exhaust valve, we have a temperature exceeding 1,400, but we have an internal piston temperature approximately that of the oil, being some 1,000 degrees less, we have tremendous heat flow through the upper cylinder and head. This heat has to be continually and efficiently rejected to avoid heat-related damage. Since cylinder design can only change in two basic respects regarding heat dissipation, wall thickness and fin efficiency, most of the heat and related component expansion/contraction issues are probably related to the design and cooling of the head if the oil cooler is properly sized. Across the head, we have very cool air entering on the intake side and very hot exiting on the exhaust side with a temperature difference of maybe 1,400 degrees. Some of the heat flow is into the intake gasses, not an entirely bad thing. But on the exhaust side, we have this very hot gas stream flowing from the upper cylinder, contacting the exhaust valve, and pulsing through the exhaust port and entering the exhaust pipe. So, in my thinking, the issue is how to maximize the efficiency of heat rejection on this exhaust side after the heat is contained in the head and exiting the exhaust port into the exhaust. This has two major components: internal and external.
Internally, I'd want the exhaust side to be as smooth as possible, ported and polished to remove all ridges and fins that can absorb and transfer heat inwardly to the engine. Polishing reduces surface area available for heat absorption. The idea being to get the maximum amount of heat outside of the engine before the head can absorb that heat. Head fins and engine oil dissipate heat that is absorbed by the head and not exited though the exhaust pipe. Probably a good head design would increase the size of the exhaust port to the maximum possible, which would cool the exhaust gasses exiting the cylinder by expansion. At this point, we are still inside of the head with almost all of the exhaust gas heat. The next step would be to move that heat outside of the head and into the exhaust pipe by two methods. The first would be to use the largest exhaust pipe possible with the least restriction. If the exhaust pipe connection to the head, at least the pipe diameter, is larger than the internal head exhaust port diameter, the gasses will expand again and that will have two effects: first being cooling as an expanding gas, and the second being lower exhaust gas pressure after the head as a result of the gasses expanding into the pipe and cooling. This expansion will also have a scavenging effect, tending to pull hot gasses out of the engine. (All of this discussion is for the non-turbo engine). The scavenging function of cooled and expanded gasses is of course limited by the "closed end" of the head nearest the exhaust valve seat. In this valve-seat area we would want maximum material thickness to retard heat flow into head and the engine. Porting & polishing before and after pics: http://www.aircooledtech.com/exhaust_pp/
The next-to-last component is header design. Looking at how most of us have our exhaust routed, I think much of the heat and related expansion/loosening issues are related to exhaust header design. Most of us have no real design. Just bent tubes stuck onto the exhaust port and directed down. I think most of the bends are too sharp and and the pipes are too small to achieve anything but rudimentary heat rejection, which results in unnecessarily high head temperatures. Some seem the same size as a 1600 on an old Beetle putting out 45 HP. Engine cooling would benefit from both large exhaust pipes and 4-1 tuned exhaust that is optimized to scavenge the exhaust. Scavenging sucks waste heat out of the engine. Here is some discussion on header design for racing engines:
http://www.geneberg.com/cat.php?cPath=15_445
http://www.epi-eng.com/piston_engine_technology/exhaust_system_technology.htm
A collector, ditch the springs!: http://vwparts.aircooled.net/Bugpack-Off-Road-Racing-Collector-1-5-8-2050-22-p/2050-22.htm
Lastly, the idea of ducting the cooling air around the heads, cylinders, and the exhaust using baffling needs some major consideration. Most of the ducted engine cooling air needs to first cool the heads with a lesser flow to the cylinders. Most baffling just directs flow to the fins in general without discriminating between the heads and the cylinders. Once that fin cooling ratio is determined and solved to reject the maximum retained engine heat, the exhaust pipes within the cowling need to be cooled for two reasons: to cool the gasses within the pipes to aid in heat extraction from the engine, and secondly to reduce heat within the cowling, which will reduce engine heat load.
By engineering to reduce engine heat absorption by maximizing engine heat rejection, at least some of the issues related to thermal expansion and head failure can be reduced or solved. Also, an engine that operates with less heat strain should last longer and should have less propensity to fail at higher power outputs.
Anyway, this is my 2¢.
Thaddeus
V/R
Smokey
Last edited:
That 2180 should work well for you! I still have a set of plans for the Sonerai II, every now and then I think about buying some tubing and getting to work. 
