dtoelke Posted February 5, 2015 Report Share Posted February 5, 2015 Hey ya'll, I just purchased a '79 M20K and had the automatic wastegate installed prior to taking possession. I got off the phone with the mechanic a few minutes ago and he mentioned that the engine was still very easy to overboost. He said he called Merlyn and they told him that is normal and the plane still needs to be operated by the POH. I was under the impression one of the primary reasons to install the wastegaste was to prevent overboosting on takeoff. I have a background in automotive work and automatic wastegates are supposed to do just that... re-route exhaust flow once a pre-determined pressure is reached. Unless the wastegate is fully open then in that case you can't re-route any additional flow and can still overboost... Could somebody clarify this? Did the shop possibly miss something on the installation or am I just mis-informed? Looking forward to joining the Mooney community, -Don Quote Link to comment Share on other sites More sharing options...
schule Posted February 5, 2015 Report Share Posted February 5, 2015 The Merlyn will not stop an overboost. You must control MP with throttle. The Merlyn does a good job of stabilizing upper deck pressures (i.e. MP) at a given throttle setting vice the old adjustable bolt, but even an airspeed change can affect MP inflight. The fully automatic boost controls were installed in the MB and SB (252/Encore). Quote Link to comment Share on other sites More sharing options...
Dave Marten Posted February 5, 2015 Report Share Posted February 5, 2015 Don, Welcome. The Merlyn does not prevent over boosting on takeoff. You will still need to smoothly set MP with your throttle on takeoff (no firewall slams). The merlyn waste gate raises the critical altitude meaning you can hold MP higher and will develop equivalent power at lower MP than the fixed waste gate. Best addition to the 231. Put 400hrs on my 231 before moving on and the merlyn and intercooler are the two best mods for the 231. http://www.merlynproducts.com/blackmagicm.html Congrats on your new bird! Quote Link to comment Share on other sites More sharing options...
dtoelke Posted February 5, 2015 Author Report Share Posted February 5, 2015 Thanks for the quick response! So I guess "automatic wastegate" is a bit of a misnomer. More semi-automatic than anything. Already had an intercooler (Turbo Plus). Looking forward to picking it up this Saturday! Quote Link to comment Share on other sites More sharing options...
manu damaschin Posted February 5, 2015 Report Share Posted February 5, 2015 Don...congratulations for the aquisition and welcome to the mooney club! After few hours and takeoffs you will see that it is not very complicating to take care of MP during takeoff roll...I had the same concern In th begining Quote Link to comment Share on other sites More sharing options...
DonMuncy Posted February 5, 2015 Report Share Posted February 5, 2015 Manu, Elaborate please. I have considered a Merlyn, and like Don, assumed you could push full throttle at take off and it would keep the MP under control. If that is not the case, what benefit does it give. Thanks Quote Link to comment Share on other sites More sharing options...
dtoelke Posted February 5, 2015 Author Report Share Posted February 5, 2015 Primary benefit is MP consistency. The fixed nature of the stock wastegate means that the orifice through which the exhaust gases circumvent the turbine flow path is fixed. On the other hand, a variable wastegate such as the Merlyn is able to actively modify the size of the orifice to allow more or less gas to pass through. So, once you've set your manifold pressure it will remain relatively constant at the same throttle position (until the wastegate limits are reached). Imagine your are taking off and set the MP to 36". With the stock wastegate you would have to continuely advance the throttle as altitude increases to maintain that 36" of MP. With the Merlyn, the wastegate orifice will instead slowly reduce in size to divert more of the exhaust gases through the turbine and maintain a constant MP. This also results in a significantly higher critical altitude since the wastegaste can continue to reduce well past the size of the stock orifice (most wastegates can close completely but I'm not sure about the Merlyn) to keep the turbine spinning at high altitudes. Other side benefits include lower heat stresses at low altitudes (since a greater volume of gas is diverted around instead of through the turbine) as well as slower turbine speeds. The primary reason I decided to get it immediately after purchase is to enable easy LOP operation. LOP is possible on the stock wastegate but can be a pain because as you induce control looping. For example, when you pull the mixture back the pressures in the exhaust manifold change which in turn causes a change in the turbine speed, which causes your manifold pressure to stray from the intended setting. So getting LOP set up requires a lot of back and forth with the throttle and mixture control until the desired setting is reached. With the Merlyn simply set the MP and pull the mixture back. The wastegate adjusts for the change in exhaust manifold pressure and you're good to go! 2 Quote Link to comment Share on other sites More sharing options...
FBCK Posted February 6, 2015 Report Share Posted February 6, 2015 I have the same setup, when pulling the mixture to lop, you will get a drop in mp and you will have to add more throttle to maintain the same mp as before you pulled the mixture back. You also have to add more throttle as you gain altitude, this with the merlyn wastegate, the wastegate in the 252 does not need such adjustment. Quote Link to comment Share on other sites More sharing options...
chrisk Posted February 6, 2015 Report Share Posted February 6, 2015 From the Merlyn literature, it is "designed to compare upper deck pressure with manifold pressure at part throttle, and adjust the waste gate according to a predetermined schedule." In other words, the Merlyn controls the waste gate so the turbo only blows as hard as needed to maintain manifold pressure. This differs from the factory fixed waste gate, where the turbo blows to hard at low altitudes, and not hard enough at higher altitudes. I suspect the algorithm is roughly: adjust the waste gate to make the upper deck pressure 2 inches more than manifold pressure. In theory, you should see a higher service ceiling, lower temperatures, and improved fuel efficiency (except at one altitude). You still set the manifold pressure with the throttle, and if you change altitude, you will need to readjust the MP. And you can over boost! P.S. Upper deck pressure is the other side of the throttle plate. One side is manifold pressure, the other is upper deck pressure. See http://www.aviationpros.com/article/10388911/boosting-your-knowledge-of-turbocharging-part-ii-valves-and-controllers Quote Link to comment Share on other sites More sharing options...
jlunseth Posted February 6, 2015 Report Share Posted February 6, 2015 We have had many discussions about the Merlyn before. THE MERLYN IS NOT AN AUTOMATIC WASTEGATE, which most people understand to be an absolute pressure controller. It is a differential pressure controller, meaning it attempts to maintain a stable pressure difference between the pressures on both sides of the throttle. The purpose of a differential pressure controller is primarily to prevent bootstrapping. So when the throttle is opened in a fixed wastegate system, the engine spins up, which produces more exhaust pressure, which spins the turbine, which provides compressed air to the induction system, which causes the engine to produce more power, which causes more exhaust pressure, which causes more turbo boost, etc. It is certainly possible to set a stable MP in a fixed wastegate system, but the turbo and throttle may chase each other around for awhile. The differential pressure controller puts a stop to that. However, it is not automatic in the sense that it is up to the pilot to manage MP, the controller will not do that. If you tip the nose over and start down, for example, the ram air effect will increase your MP, the Merlyn will not stop that, all it is doing is maintaining the set pressure difference. So yes, absolutely, you can overboost the engine in any regime of flight. The Merlyn does have one other benefit. In a fixed wastegate TSIO-360-GB or LB the wastegate never fully closes. It is set to a certain opening on the ground and always wastes some exhaust. The result is a low critical altitude compared to the Merlyn equipped engine. The Merlyn will fully close the wastegate for you, giving you a critical altitude in the 22,000 range, instead of the 15,000 of the fixed wastegate system. There is a "however". If you elect to fly that high and do not have an intercooler, you need to be careful of the Compressor Discharge Temperature, which redlines at 280F, and you do not want to exceed that redline because you will induce detonation. Absent an intercooler, you will typically run into that redline, depending on the day, at somewhere around 18-20,000 in my experience. Quote Link to comment Share on other sites More sharing options...
dtoelke Posted February 6, 2015 Author Report Share Posted February 6, 2015 Yikes guess I really did not understand that correctly! Thanks for clearing things up. I'm still surprised given how common they are in the auto world why Merlyn didn't just design an automatic wastegate.... Quote Link to comment Share on other sites More sharing options...
carusoam Posted February 6, 2015 Report Share Posted February 6, 2015 You've come to the right place... Go MS! -a- Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.