engine bay temperature

[Phoenix of Hamble]

I've got a fan in my engine room to extract warm air, but its been wired up through a switch, which is not a good option IMHO. I intend to wire it up to the alternator output, or the ignition circuit maybe if the wiring is man enough to support the fan, so that it only runs when the engine is running... and also, as the fan is quite audible in the cockpit, I also intend to wire it through a thermal switch to cut unnecessary running down.

The switches are pence each, so not a big deal, but are available in 5 degree increments from 30 degrees upwards.

What is the general consensus on the right temp switch to get?

I was thinking 40 deg C.... but not sure if that's the right answer. Its a biggish engine room, so would take a while to get up to that kind of temperature under normal weather conditions.

 

[superheat6k]

The air drawn by the engine will far exceed that of most engine bay fans, so whilst the engine is running the fan effect will likely be minimal, but important to have an overrun for several minutes after engine shut down.

I would suggest a thermostat kicking in at say 35oC and out at 30oC. Having a differential prevents nuisance stop starting around the setpoint, and most mechanical switches have a natural hysteresis anyway to provide the differential.

 

[Niffler]

The air drawn by the engine will far exceed that of most engine bay fans, so whilst the engine is running the fan effect will likely be minimal, but important to have an overrun for several minutes after engine shut down.

I would suggest a thermostat kicking in at say 35oC and out at 30oC. Having a differential prevents nuisance stop starting around the setpoint, and most mechanical switches have a natural hysteresis anyway to provide the differential.

I honestly thought about that, but on my Trawler style mobo I had until last year with a single 84hp unit, I started using the engine fan and the engine temp dropped on hard running by about 5 or 6 degrees. I assumed that the engine drew in way more air than the fan extracted, but by putting the fan extraction next to the exhaust manifold I drew out the very hottest air - a surprising difference resulted.

 

[Phoenix of Hamble]

The air drawn by the engine will far exceed that of most engine bay fans, so whilst the engine is running the fan effect will likely be minimal, but important to have an overrun for several minutes after engine shut down.

I would suggest a thermostat kicking in at say 35oC and out at 30oC. Having a differential prevents nuisance stop starting around the setpoint, and most mechanical switches have a natural hysteresis anyway to provide the differential.

Good points. One of the advantages of a thermal switch is I can wire the fan to a supply that will remain live after the engine is shut down.

The switches I've been looking at are binary on-off at a fixed temp... the hysteris point hadn't occured to me, and you're right... a fan switching on and off every few seconds as you achieve a certain temperature would probably be even more annoying than a constantly running one!... I'll have to research the switch options a bit more thoroughly.

 

[pvb]

The air drawn by the engine will far exceed that of most engine bay fans, so whilst the engine is running the fan effect will likely be minimal, but important to have an overrun for several minutes after engine shut down.

I used to think that. But a typical small diesel in a yacht at typical cruising revs will draw in about 1500 litres a minute. An inline fan (Rule, etc, 4" ducting) will shift about 6500 litres a minute on free flow. So, depending on ducting, the fan can make a significant difference.

 

[superheat6k]

I used to think that. But a typical small diesel in a yacht at typical cruising revs will draw in about 1500 litres a minute. An inline fan (Rule, etc, 4" ducting) will shift about 6500 litres a minute on free flow. So, depending on ducting, the fan can make a significant difference.

I will partially concede, but free flow will drastically reduce to actual with just a small duct run, and add a few bends and your theoretical could be a mere fraction as actual.

I am also thinking more of the behemoths on my own boat.

For the stat a simple central heating style room stat with bimetallic strip would likely work quite well. Electronic ones with fixed operation point could be an issue, unless they have diff built in. Cheap EBAY temp conttollers are great for exhaust alarms, but not so sure for thermal control of the fan.

 

[oldsaltoz]

Our engines are in fully sealed compartments (Catamaran) with no fans and high temperatures, the air is fed in through a dorade vent on deck and fed through an 8 inch flexible pipe to bottom of the compartment.

Been like that for over 10 years and not a problem.

Good luck and fair winds.

 

[Trundlebug]

I fitted an engine bay temp readout when I fitted the exhaust temp readouts a couple of years ago.

I can therefore tell you with some confidence that the engine bay gets hotter than you might think, but it varies a lot depending upon the weather / ambient temp.

Yes, in ambients of up to about 20C engine bay runs at around 30-35C.
On warmer days up to ambient 27C, engine bay can be up to 40C.
In cooler weather (around 10C) engine bay is high 20'sC

After switching off, heat soak brings engine bay temp right up to around 50C on hot days.

Above figures are for a twin engined displacement cruiser with naturally aspirated diesels. And 2x4" extract bilge blowers.