Turbo cut in revs ?

[Colin K]

Hello,
We have a Fairline Corniche with Volvo TAMD 41a's. 200hp each. We intend to go as far South West as we can as soon as the kids break up from school from our base in Ipswich. Scillies would be amazing!! I have been given an interesting article about a chap who used to cruise massive distances and to save fuel (and the saving was quite impressive ) he used to run on one engine at a time. I am not sure if this is possible with the Corniche without going around in circles and am going to give it a try this weekend.
I am curious if there is any truth in the story that it is not good to run a turbocharged engine at certain revs if those revs are just when the turbo is just kicking in. Im not sure if this is how a turbo works but wondered if anyone has an opinion. Understand that some engines have boost pressure meters?? But mine doesnt.

Cheers, Colin.

 

[gery w]

Hi Colin I also have a Corniche on 41b's . I was told that if running for extended periods on one engine the propshaft must be locked as the gearbox would have no cooling with the engine off.
We find it better to run on both at approx 1500 rpm @7/8kts for good economy.

 

[Latestarter1]

Hello,
We have a Fairline Corniche with Volvo TAMD 41a's. 200hp each. We intend to go as far South West as we can as soon as the kids break up from school from our base in Ipswich. Scillies would be amazing!! I have been given an interesting article about a chap who used to cruise massive distances and to save fuel (and the saving was quite impressive ) he used to run on one engine at a time. I am not sure if this is possible with the Corniche without going around in circles and am going to give it a try this weekend.
I am curious if there is any truth in the story that it is not good to run a turbocharged engine at certain revs if those revs are just when the turbo is just kicking in. Im not sure if this is how a turbo works but wondered if anyone has an opinion. Understand that some engines have boost pressure meters?? But mine doesnt.

Cheers, Colin.

Colin,

Covered this in some detail only weeks ago http://www.ybw.com/forums/showthrea...ion-on-a-nanni-200hp&highlight=boost+pressure

No clue where the presumption that turbocharger 'kicks in' comes from but seems to crop up from time to time. I think this thread answers your questions.

 

[Lon nan Gruagach]

No clue where the presumption that turbocharger 'kicks in' comes from but seems to crop up from time to time. I think this thread answers your questions.

The turbo boost vs engine rpm curve will often have a steep middle section, as engine rpm enters that region power output increaes significantly more -> increasing rpm and therfore boost. The "kick in" term, whilst not accurate most certainly descibes the response felt by people. The other contributing factor is that turbos exhibit a delay, it takes a while to spin up, so moving through he steep part of the boost curve you might be requesting much higher rpm than is being delivered pending the turbo getting up to speed.

 

[omega2]

It's spinning all the time if the engine is running, keep your hands away from the intake,or lose them! and no engine will run without fuel, so use it or lose it.

 

[Latestarter1]

The turbo boost vs engine rpm curve will often have a steep middle section, as engine rpm enters that region power output increaes significantly more -> increasing rpm and therfore boost. The "kick in" term, whilst not accurate most certainly descibes the response felt by people. The other contributing factor is that turbos exhibit a delay, it takes a while to spin up, so moving through he steep part of the boost curve you might be requesting much higher rpm than is being delivered pending the turbo getting up to speed.

As I said in previous thread.......'The power surge being felt is engine passing peak torque and boost levels will be rising more steeply due to shape of prop demand curve, the real one not the calculated one off the manufacturers spec sheet.

As to suggesting 'turbo lag' effects boats simply incorrect, the propeller demand builds quite slowly even if one pointlessly 'nails' the throttles. I sometimes experience poor boost control on mechanical engines and some Yanmar electronic engines which is all about the LDA (boost control) set up. Perkins M225 and its higher output brothers are particularly soft in this regard, however even they respond to graunching the LDA just a bit. This is not turbo lag.

 

[Lon nan Gruagach]

The whole point of a turbo, or super charger is to put more oxygen in the cylinder,on an engine with carbretor(s) the fuel is governed by the air flow, so no need to respond to what the turbo is doing. With injection engines the fuel amount needs to be adjusetd some other way, namley the LDA input to the injetion pump. All it does is effectivly increse the throttle setting according to boost preessure, otherwise you will just have a leaner burn instead of enjoing all the extra air (the whole point of a boost). The end result is that when the turbo starts to spin at a speed that the impeller becomes more efficient the engine starts to increse power in a non linear and time dependent way: more boost -> more fuel -> more exhaust gas -> more boost, regardless of engine load.
Sure it gets fudged by introducing electroinics to it, but if anything they will be disguising the basic effect of adding a turbo.

A turbo knows nothing of engine load., it is only driven by exhaust gas.

 

[Latestarter1]

Seems like I will never stop repeating ‘propellers move boats engines merely turn them’.

Let me try again….If we take our hypothetical diesel engine, leave it out of gear and lift the revs to 2,000 rpm and record the boost pressure, it will likely be 1 to 2 psi.
Now we cast off we gradually increase the revs, remember from my previous posts simply think of throttle as cruise control on a vehicle however it sets the engine rpm.

As we know propeller demand load is always less than engine potential power at all speeds with exception of rated speed.

At a specific throttle settling the demand causes the governor to respond by increasing the fueling. The LDA (boost control) consists of air connection between inlet manifold is a simple adjustable aneroid device on the fuel pump consisting of a spring loaded diaphragm connected to the fuel rack.

Nailing the throttle to a certain rpm and the related governor response to the load in order to achieve the set rpm is sensed by the LDA. Fuel is heat, excess free heat spins the turbine which in turn drives the compressor side creating boost which in turn increases fueling. Turbocharger is not a passive device but part of a closed loop system.

Careful adjustment of the boost control by altering the spring tension on the aneroid alters the rate of fueling therefore speed of turbo response and can turn an unresponsive slug of an engine to a rocket ship or vice versa.

Making an engine sparkle has limitations, mechanical Cat engines generally have far more ‘get up and go’ than equivalent Volvo Penta motors which made them popular in performance applications, resulting in being a little more prone to transom sooting.

Going back to our hypothetical vessel with revs set at 2,000 rpm but now subject to propeller demand, boost is now around 10 psi.

Comparisons with gasoline engines which have no governor systems other than possibly over speed cut outs makes no sense. This is why throttle man on gasoline powered offshore power boat is such a key role.