Air Intake Temp: Does it Affect Engine Efficiency?

Hello.

After another heated discussion in a different forum about the effects of LLK modifications, I wrote down my knowledge and conclusions in a technical article.

My conclusion is that, under normal conditions, larger liquid cooling systems are unlikely to provide any significant performance improvement in TDIs.

However, one small detail still seems uncertain to me, namely whether better cooling of the intake air (e.g., 40°C instead of 50°C) under otherwise identical conditions - especially the fuel injection amount! -could increase the engine's efficiency and potentially release a meager 1 or 2 horsepower.

What do educated mechanical engineers and other scholars say about it?
Okay, bitte gib mir den deutschen Text, den du übersetzt haben möchtest. Ich werde mein Bestes tun, um eine genaue Übersetzung zu liefern.

Hey.....Do you have any textbooks available, or do you, Ulf?

The mean pressure (Pme) indicates the engine's performance (and is proportional to it).
It is calculated based on:

Pme ~ p2 * (1/lambda) * ne * lambdaL

p2: Air tightness after installation.
lambda: air-fuel ratio, amount of air drawn in.
no: effective efficiency
lambdaL: Delivery rate (depends on factors such as waste heat from the charger, etc.)

In addition to these factors, heating value and minimum air requirements are also important, but they are already considered in the power plant specifications and are therefore assumed to be met.

p2 is calculated:

p2 = r2 / (R * T2)

r2: Turbo pressure.
R: Gas constant (specifically, of air).
T2: Temperature after the compressor.

As the air density increases, the effective power output of the engine is significantly increased.

How much less air can fit into a motor's working stroke at 10°C can be calculated by someone else, but it's simple based on the ambient pressure.
I'm already feeling math-related stress today... sorry!

Hello.

Yes, the efficiency changes. The behavior of the air is more favorable for the engine process at low temperatures (higher air-fuel ratios). In a theoretically perfect engine, it's on the order of 1% if you increase the amount of air by 3% in a diesel engine under full load. However, it's difficult to say exactly what this increase in the air-fuel ratio actually achieves in a real engine (e.g., combustion process?, heat losses to the walls?).

At the time when the mass airflow sensor (MAF) limits the fuel injection amount, the temperature reduction has a double effect. Because with each boost pressure, an additional 3% more air passes through the MAF due to the cooling (from 50°C to 40°C), which further increases the fuel injection limit. Therefore, the fuel spray quality will increase by more than 3%, and the driver can definitely notice this.

I cannot say whether and to what extent the turbo lag, as you describe in your article, will increase. I think it's necessary to first perform a calculation for the entire intake system, taking into account all volume changes and flow losses. Therefore, it will depend on what the 'larger intercooler' actually looks like.

Overall, I think, like you, that simply replacing the intercooler alone won't make much of a difference. I would only do it if I also install a more powerful charger and then adjust the fuel injection accordingly. The existing system's efficiency is likely not significantly hindered by the intercooler. Moreover, because the lower intake air temperature also reduces NOx emissions, manufacturers can't afford to install an intercooler that is too small as standard equipment.

Best regards, Ernst.

Okay, I've been thinking about the following limit analysis:

The warmer the air, the more it must be compressed in order to fit the same amount of air (by weight) into the same volume. Compressing air requires energy, and therefore consumes power.

If I cool everything down to -70 degrees Celsius, I will have the same amount of air at 1 bar as I would normally have at 2 bar (and 20 degrees Celsius). Is it easier to compress air to 1 bar x 20 (meaning 20 times the pressure) than to 2 bar?


Therefore: the denser and colder the air, the less the work required for compression.

If less work is consumed within the engine itself for the same amount of fuel, does that mean more power is delivered at the shaft, or what?

'In my opinion, a larger intercooler won't bring any improvement as long as you don't make any software changes. If you assume that using a different intercooler reduces the intake air temperature by 20 Kelvin, while keeping all other parameters the same, it could even lead to a decrease in efficiency. This is simply because, while you're getting more air mass into the engine, the fuel injection remains constant. The cooler air mass only results in a reduction in exhaust gas temperature and, consequently, enthalpy. However, since the target boost pressure remains constant, the variable geometry turbocharger (VTG) control unit has to work harder to achieve that boost pressure, which increases the exhaust back pressure and, therefore, the charge exchange energy... resulting in a decrease in efficiency!'

As Ulf already wrote in his article, in terms of rated power, the mass airflow sensor (MAF) is completely irrelevant. The amount of fuel injected is only limited by the torque limitation map. The MAF only influences the fuel injection amount when the air intake is extremely low (e.g., due to a faulty MAF sensor or a leak after the MAF sensor and before the turbocharger compressor). So, as I said, both of these phenomena do not occur, which means the fuel injection is only limited by the 'torque' map. Therefore, a different MAF sensor cannot increase power unless you change the other parameters!

Another aspect that should be considered is that if you change the size of the lower radiator (the radiator located before the water pump), you should also be aware that you may negatively impact the performance of other radiators used for water cooling and climate control. In extreme cases, this can lead to the fans turning on earlier, which can result in a reduction in performance.


For example, if you add a second intercooler, you have the problem of increasing the volume of the intake manifold. Also, the elasticity increases with the additional hoses. The result: a very nice turbo lag .

So, I've come up with the following limit analysis:

The warmer the air, the more it must be compressed in order to fit the same amount of air (by weight) into the same volume. Compressing air requires energy, and therefore consumes power.

If I cool everything down to -70 degrees Celsius, I will have the same amount of air at 1 bar as I would normally have at 2 bar (and 20 degrees Celsius). Is it easier to compress air to 1 bar x 20 (meaning 20 times the pressure) than to 2 bar?


Therefore: the denser and colder the air, the less the work required for compression.

If less work is consumed by the engine for the same amount of fuel, does more power come out at the shaft, or what?


If you are trying to maintain a constant boost pressure, this approach is incorrect! However, if you want a specific amount of air in the engine at the end of the compression stroke, you can reduce the boost pressure with colder intake air, which will also reduce the exhaust back pressure. This will decrease the work required for the charge exchange process and increase the efficiency!

The engine's performance also depends on the temperature differences between the intake and...
Combustion temperature.

The greater the difference, the more power.

With intake air that is theoretically as warm/hot as the combustion temperature, one would expect...
if you can't deliver any more performance

and what 10°C less intake air temperature brings, is quite a bit .

@ulf: According to your logic, the intercooler would be superfluous and add weight?!?

If anyone is more interested, I'd be happy to send them some formulas about it .

greets Andy

Julian wrote:

Hey.....Do you have any textbooks available, or do you, Ulf? .

Hi Julian,
No, I'm not a trained engineer in the field of motors. And, frankly, I don't have enough time to read up on the subject "on the side."

Quote:

The mean pressure (Pme) indicates the engine's performance (and is proportional to it). It is calculated based on: Pme ~ p2 * (1/lambda) * ne * lambdaL p2: Air tightness after installation. lambda: air-fuel ratio, amount of air drawn in. no: effective efficiency lambdaL: Delivery rate (depends on waste heat from the turbocharger, etc.)

What exactly does "air-fuel ratio of aspirated amount" mean here?

And: Is the mean pressure already the solely decisive factor for the efficiency ?

By the way, I made a mistake in the original post: Lowering the LL temperature alone, while keeping all other conditions completely unchanged, is not possible.
Assuming a constant boost pressure, cooler air naturally carries more mass into the cylinders .

ErnstS wrote:

The behavior of the air is more favorable for the engine process at low temperatures (higher air-fuel ratios).... in a theoretically perfect engine, it is on the order of 1% if you increase the amount of air by 3% in a diesel engine under full load. However, it is difficult to say exactly what this increase in the air-fuel ratio actually achieves in a real engine. (Combustion process?, Heat losses to the walls?).

Hmm, some time ago, someone here wrote in a poetic way that a "mixture lambda" of 1 is an optimization parameter for efficiency (I myself don't know to what extent that's true).
However, if the TDI engine is still running with a significant excess of air even under full load, then by using "more effectively cooled intake air" (meaning more unused air mass), I am actually moving further away from Lambda = 1, which should decrease the efficiency.

Ben1972 wrote:

By using cooled excess air, I only achieve a reduction in exhaust gas temperature and, consequently, in enthalpy. However, since the target boost pressure remains constant, the variable geometry turbine (VTG) control device must provide more air to reach the desired boost pressure. This results in a higher exhaust back pressure, which in turn increases the charge exchange energy...and the efficiency decreases!!!

Also, another interesting aspect, and in my opinion, it's not something to dismiss outright... but as I said, I'm an uneducated here.


It certainly seems to be a rather multifaceted topic overall.
I would be pleased if we could reach a clear and definitive statement here that is supported by everyone.

However, this requires that someone eventually abandons their previously held position.
However, no one should be too proud to admit it, because it's easy to be wrong when dealing with such complex topics, and I think we should all be mature enough to acknowledge that.
a) acknowledging mistakes.
b) Not to exploit such "opportunities" to put others down.

In this spirit, I kindly request further contributions .

Hi Julian,
No, I'm not a trained engineer in the field of motors. And, frankly, I don't have enough time to read up on the subject 'on the side.'

Hm, your lack of interest is not as significant as you might think.


What exactly does 'air-fuel ratio of aspirated amount' mean here?

And: Is the mean pressure already the solely decisive factor for the efficiency ?

In the amount of air drawn into the engine, that's the desired air-fuel ratio. In short, something like 11:1.

Beim Mitteldruck gibts versch. Versions:

Internal/induced mean pressure: equivalent to the specific work acting on the piston.
Effective MD: Calculated from the motor's torque.
Frictional MD: Difference between internal and effective MD.

Consequently, the efficiency is also calculated based on the 3 MDs.

Roughly speaking, WG = Benefit/Effort.

But writing all of that down would be too time-consuming. It wouldn't be worth it.
It can be said that the mean pressure is measured in [bar], and the efficiency in [%]. The input variables are the same, but the calculation method is slightly different. Therefore, I'm simply stating that the units are very roughly different here, and thus, efficiency is approximately proportional to mean pressure.

Hmm, some time ago, someone here wrote in a somewhat poetic way that a 'mixture lambda' of 1 is an optimization parameter for efficiency (I myself don't know how true that is).



Complete nonsense, I don't believe that guy. (Because a three-way catalytic converter requires a Lambda value of 1, all gasoline engines consume 10% more fuel compared to a possible increase in the air-fuel ratio to 1.1.)
Then the curve flattens out significantly.

Otherwise, there are simply too many details that need to be considered with a TDI engine. For example, in homogeneous combustion, an intercooler has no positive effect on efficiency, and in isochoric combustion, it can even have a negative effect if the air mass is kept constant.

In a real engine, intercooling is beneficial because it allows the engine developer to increase the compression ratio, while still preventing the peak stress from becoming too high when the intake air is cooled.

We can't do that, so while the slight increase in airflow might improve efficiency, it certainly won't be noticeable. Unless the mass airflow sensor is currently limiting the fuel injection. But for that to happen, the larger intercooler really needs to bring about a significant temperature reduction. And it will only do that if the vehicle manufacturer previously installed a very small intercooler, for some reason.

Best regards, Ernst.

Okay, let me share some of my humble practical knowledge.

I'm a T4 owner, so I usually spend my time on the T4 forum.

Some ACV drivers (75kW TDI) have replaced their standard intercoolers with the intercooler from the AXG/AHY models (111kW). The latter is significantly larger and sits directly in front of the radiator.

The smaller, mass-produced ACV heat exchanger is located in the engine compartment and is only indirectly cooled.

'Everyone who has upgraded is experiencing a noticeable increase in performance, although the power delivery is slightly delayed – likely due to the increased volume.'

'Personally, on my last T4 (a 65kW TDI model without a turbocharger intercooler), I retrofitted the intercooler from an ACV engine and noticed similar improvements. In particular, I was able to maintain a higher cruising speed on the highway when using the cruise control.'

The performance measurement was also positive (74 kW, 220 Nm - stock values are 65/195), but unfortunately, I only measured with the LLK (Low Pressure Fuel System). Therefore, I don't know the 'before' value. Since most TDI engines tend to vary slightly, the actual increase from the LLK modification might not be huge, but it was noticeable.

I believe that the larger charge air temperature sensor affects the injection quantity, or am I wrong?

I hope I have contributed something .

... the pure llk increase won't be that huge. But it was noticeable.

I think that the intake air temperature sensor affects the fuel injection amount } due to the higher intake air temperature, or am I wrong? }

Well... that wouldn't be an increase in efficiency if the fuel injection amount is increased. Of course, it's possible that such an adjustment is taking place. And that would also explain why there might be power limitations in the summer and increased power in the winter. However, the engine control unit would then have to change the torque limitation, or, in addition to the three limitations, there would have to be an additional fuel injection adjustment.

Best regards, Ernst.

maniac wrote:

Some ACV drivers (75kW TDI) have replaced their standard LLK (low-temperature coolant radiator) with the LLK from the AXG/AHY models (111kW). This one is significantly larger and sits directly in front of the water cooler. All those who have been upgraded

are experiencing a noticeable increase in performance, with a slightly delayed boost - likely due to the larger volume.
Hi.

"Sorry, but the word 'spüren' is too vague for me in this context. If the later turbo boost kicks in more forcefully at the same time, it can subjectively make it feel like there's more horsepower."
It would be interesting to, for example, perform DZR measurements before and after under otherwise identical conditions.
Or reliable

Quote:

test stand measurements.

In my personal experience, on my last T4 (ajt 65kw TDI - no LLK), I retrofitted the LLK (low-pressure cooler) from an ACV engine and was able to observe similar results. Specifically, on the highway, a higher cruising speed could be maintained using the cruise control.
The performance measurement was also positive (74 kW, 220 Nm - standard: 65/195), but unfortunately, I only measured with LLK (Low Load Kit). Therefore, I don't know the "before" value. Since most TDI engines tend to vary slightly, the actual LLK increase might not be that huge. However, it was noticeable.
OK, but your experience (previously without a large language model, now with a large language model

Quote:

) somewhat misses the actual topic (= previously a smaller large language model, now a larger large language model). I can certainly imagine that, without any exhaust gas recirculation (EGR), virtually every oxygen atom reported by the lambda sensor is used for (still relatively clean) combustion, and that the torque or pedal limitation usually occurs above the soot limit. With a reasonably effective exhaust gas recirculation (EGR) system, the soot limit will be significantly higher. However, this would inevitably lead to an increased fuel injection volume, which would be the primary cause of the increased power output. Based on your experience, it's not possible to conclude, at least initially, that there is a higher efficiency (i.e., more power output with the same amount of fuel injection).

I believe that the charge air temperature sensor, }through its measurement of a higher charge air temperature, }affects the fuel injection quantity, or am I wrong?}
In detail, yes. Because the possible injection amount is calculated based on the mass airflow sensor (MAF) signal, but not based on the coolant temperature.

The maximum injection quantity is primarily limited by the torque curve at the rated power point. Most modern diesel engines operate at the rated power point with a smoke level of approximately 1. Smoke becomes visible at a level of 2.5. This means that the smoke limit only comes into effect much later and is only a safety feature in case of an air leak upstream of the ATL compressor. Therefore, it is not entirely correct to say that the injection quantity is calculated solely based on the mass airflow sensor (MAF).

If you want to achieve a performance increase with a different intercooler without changing anything else, it's better to use an intercooler that reduces pressure drop while maintaining the same cooling capacity. This way, the turbocharger has to generate less boost pressure. The boost pressure is only measured shortly before the intake manifold, and therefore after the intercooler. If the pressure drop across the intercooler is reduced, the turbocharger doesn't have to build up as much boost pressure. For example, if the target boost pressure is 2400 mbar and the pressure drop across the intercooler is 150 mbar, the turbocharger has to generate 2550 mbar of boost pressure. If the pressure drop across the intercooler is only 50 mbar, the turbocharger only has to generate 2450 mbar. This means the variable vanes don't have to be adjusted as much, the exhaust backpressure decreases, the work required for the charge exchange decreases, and therefore the efficiency increases.

Ben1972 wrote:

The maximum injection quantity is primarily limited by the torque map at the rated power point. Most current diesel engines operate at the rated power point with an approximate smoke number of 1. Smoke becomes visible starting at a smoke number of 2.5. This means that the smoke limit only comes into effect much later and is only a safety function in case of an air leak upstream of the ATL compressor. Therefore, it is not entirely correct to say that the injection quantity is calculated based on the mass airflow sensor.

Hi Ben,

I feel the same way about the 1.9 TDI.
But if maniac achieves a tangible and measurable increase in performance through the installation of an LLK, then I assume that something significant has really happened.

And the most obvious explanation, in my opinion, is that his car, without any exhaust gas recirculation (EGR), was running "right on the edge of soot production" at full load, in order to avoid wasting any of the fresh air that wasn't readily available.

If the injection quantity was already limited by the speed control even without the LLK (Low-Pressure Fuel Pump), then the LLK shouldn't have been able to provide any additional performance .

Quote:

If you want to achieve a performance increase with a different intercooler without changing anything else, it is more appropriate to use an intercooler that reduces pressure drop while maintaining the same cooling capacity. In this case, the turbocharger has to generate less boost pressure. This boost pressure is only measured shortly before the intake manifold and therefore after the intercooler. If the pressure drop across the intercooler is reduced, the turbocharger does not have to build up as much boost pressure. For example, the absolute target boost pressure is 2400 mbar, and the pressure drop across the intercooler is 150 mbar... the turbocharger has to generate 2550 mbar of boost pressure. If the pressure drop is only 50 mbar, the turbocharger only has to generate 2450 mbar. This means that the variable vanes do not have to be adjusted as much, the exhaust back pressure decreases... the work required for the charge exchange decreases, and thus the efficiency increases.

That's an interesting thought.
However, an LLK (likely referring to a larger intercooler) would probably need more or larger charge air ducts, which would likely make it physically larger, potentially causing installation problems with the vehicle's bodywork.
So the question is whether the achievable energy savings or the increased performance (at full load) would justify the effort... Can you estimate roughly what the "benefit" might be?