Influence of the T ° of the intake air on the efficiency

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Pierre-Yves
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Influence of the T ° of the intake air on the efficiency




by Pierre-Yves » 08/03/08, 11:27

on a boat, the temperature in the machine room is always very high and very often reaches 45-50 °. To lower it, you need fans stirring around 10 000 m ^ 3 / h (for a boat around 16 meters). There is often a fan in admission and another in extraction.

I am looking for the relationship between the temperature of the intake air and the efficiency of a diesel engine. On the manufacturers' instructions, it is stated that the temperature of the machine room must be between 20 and 30 °, and that it must not exceed 50 °.

So I'm looking for more specific information. All I know is that on a boat the improved ventilation has saved 10%.

Thank you !!!!!
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by Chatham » 08/03/08, 11:54

Simple: hot air has a lower density than cold air, less density = less filling, therefore less power ... this is a phenomenon well known to airplane pilots: when you forget heating (hot air to prevent icing of the carburetor when rolling on the ground or downhill), the power drops significantly ...

On my boat, the air will not be sucked into the machine box (which must also be ventilated to avoid engine hot spots and fuel heating) but by a bream box (air intake separating the water from the fuel). 'air)
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by Pierre-Yves » 08/03/08, 12:14

Chatham wrote:when we forget the heating ..., the power drops significantly

Thank you, Chatam, but I need more !! this remains qualitative. What I'm looking for is style numbers: with 20 ° intake, the engine has an efficiency of x%; with 50 °, it goes to y%.
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by Flytox » 08/03/08, 21:22

Hello Pierre-Yves

The answer below taken from "The preparation of engines" by Luc Méloua:

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by Other » 08/03/08, 22:23

Hello

A subject often debated on forum which leads to interpretation
between power and efficiency (again)
How do you interpret these curves?
Image


on a boat, the temperature in the machine room is always very high and very often reaches 45-50 °. To lower it, you need fans stirring around 10 000 m ^ 3 / h (for a boat around 16 meters). There is often a fan in admission and another in extraction.


Regarding boats, it is imperative to have an engine compartment ventilation system and even more efficient on gasoline engines, before touching the starter it is necessary to leave the ventilation to eliminate any fuel vapor in this closed room, a simple spark and it could be the explosion
besides the starter motor and the marine alternator are spark proof, the exhaust manifold and cooled with water

Taking the air outside gives maximum power gain that's all, it takes a baffle box to keep the engine from drinking a cup of water, many boats shoot directly into the ventilated compartment.

As for planes if we draw hot air from the carburetor (carbheat) there is a drop of 150rpm, it is normal we suck a smaller mass of air, with a carburetor we are also too rich with hot air,
In reality there is a loss of power only if the throttle grip is deep, just as there is a loss of maximum power with the air filter (on seaplanes, the air filter is removed to gain 100rpm on takeoff)
If an airplane flies in level 75% of the power without touching the throttle grip, pull the heater, the engine loses 150rpm, just increase the throttle grip and readjust the mixture ratio, it loses nothing at all, on the contrary it works more economical.

the only drawback is that the cylinder head temperature increases slightly.

If the temperature of the air swallowed increased the efficiency, when we flew -30 it should become super powerful and economical
and this is not the case on a gasoline carbureted engine.

My diesel vehicles, never in the middle of winter -25 full throttle I managed to reach the full gas speed of the summer, there is more than 10kmh of difference

I fully agree that full power, when the air swallowed is cold the available power increases.
but at cruising power that does not change anything, reduce by reducing the accelerator or by consuming hot air?
if the ratios are well adjusted ..

Andre
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by chatelot16 » 09/03/08, 01:41

do not confuse performance and power

hot air is less dense than cold air: therefore warmer intake air will directly reduce the maximum engine power

but on a boat engine in cruising power we are not at the maximum and that is not the problem

if unfortunately we approach the maximum power the result could be serious: if the maximum flow rate of the injection pump is set for cold air, with hot air the diesel will not burn completely and will be wasted in smoke black

more reasonably, the hotter intake air will raise the temperature at the end of compression in the cylinder and as the temperature of the cooling water is constant there will be a little more loss of heat by cooling, or a little drop in efficiency: but I do not see how to calculate it in a general case without taking into account the exact characteristics of the engine and even I believe that it would be easier to measure than to calculate ...

I think that if the temperature of the cooling water rose exactly with the ambient temperature, the yield would be constant: but not necessarily the lifespan

this result has been verified with air-cooled engines which withstand heat well: with water cooling, we are limited by the boiling temperature of the water
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by Chatham » 09/03/08, 09:38

chatelot16 wrote:
more reasonably hotter intake air will raise the temperature at the end of compression in the cylinder

this result has been verified with air-cooled engines which withstand heat well: with water cooling, we are limited by the boiling temperature of the water


Not at all: the hot air at the intake lowers the temperature at the end of compression, again for a density reason which is essential ... this in all phases of operation ... which increases consumption since for the same power it is necessary to "give more gas", therefore more flow to the injection pump (assuming that it is a simple diesel, not an ultra sophisticated HDI ...)
On average, the efficiency of water-cooled engines is much better than that of air engines, because in fact the average INTERNAL t ° of the engine is significantly higher on a water engine ... which we cannot afford on an air motor on which the temperature variations are very important ...
When I spoke of efficiency with cold air, it is cold air at positive t ° because with air at -20 -30 for example, the combustion t ° drops which reduces the efficiency because less expansion of gases.
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by Pierre-Yves » 09/03/08, 11:57

thanks to Flytox for his equations and to André for his curves, and shame on me for the confusion between power and efficiency !!

I forgot to say that I posed the problem for trawlers at work and not for a cruising boat. It is when they trawl that these boats consume a lot (from 120 to 180 liters per hour ...). And they spend about 2/3 of their time trawling. Even if progress is being made on trawls, which are more selective, less devastating and more economical, consumption represents 33% of turnover. Hence the virtual bankruptcy situation of this sector, the temptation to practice the vicious circle of overfishing, to go out to sea when the weather conditions say it would be better to stay at the bistro, etc.

Any gain is therefore appreciable. I know the effect of temperature on the density of the air and I wanted to have figures: it is more telling.
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by Other » 09/03/08, 15:56

Hello
Not at all: the hot air at the intake lowers the temperature at the end of compression, again for a density reason which is essential ... this in all phases of operation ... which increases consumption since for the same power it is necessary to "give more gas",


I speak for a gasoline engine
Possibly the theory dictates this, but the next time you get on a Lycoming or Continental bird.
equip a Thermocouple on the cylinders (or at least cylinder No4) if you also have (an EGT although not necessary for this test)
Keep it cruising Rpm stabilized without the carb heat raise the temperature of the cylinders, then not 5 minutes but a 15 to 20 minutes operate with the Carb heat (heater) observe the temperature of the cylinders, afterwards you will explain to me

Why do you think Lycoming doesn't recommend running continuously on the heater, not for power, at -20 I often run continuously on the carb heat (yet the air is as dry as a nail at these temperatures.
When the cylinder head becomes too hot, those with high compression tend to self-ignite (even at 100LL)

Andre
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