Preventing abnormal combustion

[jamsbong]

There’s also the consideration of laminar vs. turbulent flow through the radiator.

Yep

[riff_raff]

When comparing the pros/cons of intercooling versus DI, you must consider the net energy balance resulting from each. With an air/air heat exchanger used for intercooling, there is a transfer of energy to the cooling airflow. With DI, energy is only transferred from the air to the fuel mass, but there is very little energy lost in the process.

[Tommy Cookers]

As for intercooling, detonation prevention isn't the reason it's there.
Intercooling is more of a benefit to pre compression. That's my rule of thumb anyway. If there's no compression next in a cycle feel free to heat things up.
If there's a compression stage next, then inter cooling is the way to go. Things need to be as cool as possible between compression stages.
Theoretically if fuel is injected after compression of the air, then it should be injected hot; peak temps should increase after combustion (but as riffraff said, it's all empirical anyway.
Port inject and some DI is before compression is complete, so this favours cool injection i suppose.

AFAIK intercooling meant cooling between stages of supercharging
eg aircraft engines had such charge cooling only when they had 2 stage supercharging
there was anyway evaporative charge cooling via very rich mixtures at full power, always upstream of a supercharger
(such 'turbocharged' engines actually had a turbo 1st stage and a mechanically driven 2nd stage, too much or too little (air/air) intercooling could occur (P-38) as the turbos did little work at low altitude)


modern DI has (largely) late injection to minimise exposure if fuel to compression process, so avoiding detonation
aircraft spark ign DI also had late injection, continuing well after the sparking
(some 1950s 2 stroke/2 cycle cars and modern scooters, outboards etc use late injection to start after exhaust port closure)
until recent years 4 stroke/cycle production (and some race) car spark ign DI was early, around btdc ie before compression

a high-rpm race engine will spark 45-65deg before tdc, so won't such DI be injecting before much of the compression ?
unless we are (in part?) injecting after the spark ??
this seems possible, is it desirable ??

[olefud]

When comparing the pros/cons of intercooling versus DI, you must consider the net energy balance resulting from each. With an air/air heat exchanger used for intercooling, there is a transfer of energy to the cooling airflow. With DI, energy is only transferred from the air to the fuel mass, but there is very little energy lost in the process.

Very good point. If the fuel is to burn it’ll have to evaporate somewhere along the line.

Generally, there are two concurrent advantages to intercooling; a/ providing a more stable combustion mixture and b/ a denser gas. The latter was rather important for WWII two-stage boosting and early on poor fuel, but the former is more germane to the fuel flow rate conundrum. While turbocharging can provide excess air, with fuel as the limiting factor, a leaner, more stable mixture can allow higher RPMs. It will be interesting to see which way the compromises go.

[riff_raff]

.....Generally, there are two concurrent advantages to intercooling; a/ providing a more stable combustion mixture and b/ a denser gas. The latter was rather important for WWII two-stage boosting and early on poor fuel, but the former is more germane to the fuel flow rate conundrum. While turbocharging can provide excess air, with fuel as the limiting factor, a leaner, more stable mixture can allow higher RPMs. It will be interesting to see which way the compromises go.

Actually, the primary benefit of using charge cooling between compression stages is that it improves the performance of the secondary compression device. With regards to SI combustion, I believe lean mixtures typically have less combustion stability than stoichiometric or slightly rich mixtures.

[ringo]

Actually, the primary benefit of using charge cooling between compression stages is that it improves the performance of the secondary compression device. With regards to SI combustion, I believe lean mixtures typically have less combustion stability than stoichiometric or slightly rich mixtures.

Yep, aka less work from the pistons on the compression stroke.
To add to that, the thermal efficiency doesn't improve, since the temperature in the cylinder is lower, which affects peak temperature, but if the fuel is preheated before it enters the cylinders there could be an efficiency improvement.

I think the Honda engine in the turbo Mclaren had the fuel preheated?

Teams may be able to do this if they maybe wrap a coil of fuel pipe around the exhuast nozzle. Haha.

[olefud]

.....Generally, there are two concurrent advantages to intercooling; a/ providing a more stable combustion mixture and b/ a denser gas. The latter was rather important for WWII two-stage boosting and early on poor fuel, but the former is more germane to the fuel flow rate conundrum. While turbocharging can provide excess air, with fuel as the limiting factor, a leaner, more stable mixture can allow higher RPMs. It will be interesting to see which way the compromises go.

Actually, the primary benefit of using charge cooling between compression stages is that it improves the performance of the secondary compression device. With regards to SI combustion, I believe lean mixtures typically have less combustion stability than stoichiometric or slightly rich mixtures.

I think we’re saying the same thing; the denser mixture is important for two-stage boosting.
And I agree that a richer mixture is the better go; but with fuel metering, getting a lean mixture to burn well will allow for a few more RPMs.

[ringo]

Olefud, when you say two stage boosting, what do you mean?

[olefud]

Olefud, when you say two stage boosting, what do you mean?

Just that a first supercharger feeds pressurized air to a second supercharger in series to gain a greater total pressure boost. There’s a point of diminishing return for boost in a single supercharger. It was common for high altitude aircraft engines during WWII.

[Tommy Cookers]

and some GP cars 1938/9 (I think), certainly F1 Ferraris and Alfa Romeos around 1950-1
with Roots type superchargers (because they would have a very limited pressure ratio/stage)

and V16 BRM
2 stage centrifugal I think
5 bar gauge

[flynfrog]

it was also common on older GMC diesel engines to turbo and supercharge

[ringo]

Olefud, when you say two stage boosting, what do you mean?

Just that a first supercharger feeds pressurized air to a second supercharger in series to gain a greater total pressure boost. There’s a point of diminishing return for boost in a single supercharger. It was common for high altitude aircraft engines during WWII.

Well you don't have to go that far really. The pistons are the second stage of compression. Hence the intercooler between the compressor and engine.
So in essence, the denser air is important for even single stage boosting in a piston engine, as the pistons are the second compression stage. An intercooler is basically a must have if maximum power is to be considered.

[Tommy Cookers]

The pistons are the second stage of compression. Hence the intercooler between the compressor and engine.
So in essence, the denser air is important for even single stage boosting in a piston engine, as the pistons are the second compression stage. An intercooler is basically a must have if maximum power is to be considered.

charge cooling is not primarily to benefit air density (ie massflow, hence power)
what dominates maximum power is boosting massflow by (relative to fuel quality) lowering the CR to allow increased supercharge (also some diesels do this)
this is what the supercharged aircraft engine was all about
(a small sacrifice in TE but a big gain in power, also the ME is good, like 'downsizing')

evaporative cooling was and is a charge cooler anyway (although rich mixture benefits are measured independent of cooling)
BTW charge cooling positioned as you describe was (and is ?) called aftercooling

reducing compression work by pistons has minor benefit to power but is nice, that's why most GDI partly injects early

[riff_raff]

It is always best to perform as much compression/expansion work as possible within the reciprocator, since it is a very efficient device for doing this work. The only reason for using a turbocharger to perform additional compression/expansion work of the intake/exhaust gas, is because the turbo system would have lower overall weight, and/or could give better performance under an extreme range of operating conditions.

[ringo]

Yes, the pressure ratio seen by the reciprocating compressor/piston cannot be achieved easily by the turbo/supercharger.

charge cooling is not primarily to benefit air density (ie massflow, hence power)
what dominates maximum power is boosting massflow by (relative to fuel quality) lowering the CR to allow increased supercharge (also some diesels do this)
this is what the supercharged aircraft engine was all about

@ Tommy What do you mean by increased supercharge?