So can we generally say like this.
Turbo diesel and GDI Turbo Gasoline does not get much effected on power output from
Normal Port injection Turbo Gasoline engines! Is that right?
Plenty of older diesels with mechanical fuel injection have a governor that delivers a fuel volume per stroke that is dependent on accelerator position or governor operation, but is NOT sensitive to intake temperature or pressure. Air intake is unregulated and the fuel delivery is set such that under the worst foreseeable ambient temperature and pressure conditions, it is still lean. Those engines will make torque/power in response to fuel delivery and will be insensitive to intake air temperature and pressure.
Plenty of turbocharged have boost pressure regulation that is based on absolute pressure. At higher altitude it just works the turbo harder. (Higher pressure ratio) It will be relatively insensitive to intake air pressure. Even if it is an old style fixed-pressure wastegate, it's less sensitive to intake air pressure. Example 14.7 normal atmospheric + let's say 10 psi boost pressure = 24.7 psi absolute intake pressure. High altitude ... 11.7 psi barometric pressure (20% reduction) + the same 10 psi boost pressure = 21.7 psi absolute intake pressure (12% reduction). This can apply to turbo gasoline and diesel engines - if it has an old school spring loaded wastegate, this is more or less how it's going to work.
Plenty of newer emission-controlled diesels have MAF sensors, MAP sensors, IAT sensors, wide-band O2 sensors in the exhaust, apply correction factors for the boost pressure regulation at low barometric pressure (high altitude) in order to avoid overspeeding the turbo, adjust fuel delivery in accordance with all of this, and will thus vary their torque/power output in response to barometric pressure and temperature just like a spark ignition engine will.
To back up a few steps - a gasoline engine (unless direct injected) is sensitive to the compressed mixture - since it is a mixture, it can ignite before it should, or it can detonate instead of burning progressively.
A diesel never has a 'mixture', exactly - the fuel is supposed to ignite as it is injected and burn as it is injected.
Ok, there tends to be a wee bit of lag before the injected fuel starts burning, and it takes a bit of time for all of it to burn - but you can't really get detonation. And the hotter the chamber, the quicker the fuel should start burning as each injection event begins.
That's why diesels love turbochargers and don't care about hot charge air too much.
If they are intercooled, it's probably more to increase density so more mass gets through the valves into the chamber.
In a gasoline engine, too high a pressure/temperature in the chamber means detonation, causing destruction of vital and expensive parts.
Well a diesel by definition is a detonation engine as it is compression ignited, apart from cold starting. The other issue is diesel fuel is MUCH harder to ignite, just poor some on the ground & try to light it with a match ! Try the same with petrol & you better stand back or get burned with the whoosh effect
Diesels are hard to start in cold weather because its the increase in the air temperature during compression that causes the air/fuel in the cylinder to ignite. A stone cold engine in winter has a high thermal conductivity, so the first few compression cycles can see most of the heat from compressing the air absorbed by the piston & bore, leaving insufficient to ignite the fuel when injected. If cold enough & in extreme circumstances this can lead to hydro lock on a cylinder !
Petrol or Gasoline engines once above a low ambient temperature, prefer a COLD intake charge hence the use of inter-coolers & water injection etc to keep the charge temperature down. Any increase in the intake charge temp will result in higher in cylinder temps during compression stroke which if the cylinder is running near pre-ignition when on full boost may be enough to push it over the edge. This is why most turbo engines have knock sensors, to retard the ignition if detected.
Must take issue with a couple of points.
- Detonation is not the same as compression ignition and it is very misleading to refer to a diesel as a "detonation engine".
- Diesel fuel has a lower ignition temperature than gasoline. In a compression ignition engine, gasoline is harder to ignite than diesel fuel.
I think you will notice I did elaborate on the detonation/Compression ignition issue later
I agree that compressed air fuel mix of gasoline is harder to ignite, but the lower the compression the easier it is to ignite & pre-ignition often happens before high compression is achieved due to glowing components in the combustion chamber.
Compression is NOT linear in a cylinder, its exponential. By that I mean if you seal a cylinder with the piston at BDC, then move it towards TDC, it takes HALF the stroke to double the pressure once to 2bar, then half of the remaining stroke to double it again to 4bar, every halving of the remaining stroke doubles the pressure ! So if we say that 12/1 CR is difficult to ignite, but 6/1 is fairly easy, then the pressure goes from 6-12/1 in a few mm just before TDC ! That said in real gasoline engines few exceed 9/1 CR, due to valve overlap, unless running on high octane fuel. The combustion chamber volume is used to control the maximum CR achievable, in most engines
Compressing air creates heat & the heat creation is relative to the pressure applied, so the specific heat of the mixture reaches maximum at TDC on the compression stroke. So if you up the intake charge temperature, you may easily exceed the safe working temperature or self ignition temperature of the air fuel mix. Post turbo intake temperatures can exceed 90c if ambient is around 25c, this at 1.5bar boost, with a good inter-cooler this can be reduced to around 70c or less.
indicates that an ambient intake of 27c for 1000cc, compressed to 10/1 will produce an ideal gas temperature of 477c, this does NOT allow for containment cooling/heating, as in heat loss/gain to/from the components in contact with the air, depending if its a cooling or heating cycle. But you can get an idea from 10/1 compression creates 17.5/1 heat increase. Applying that to the 70c intake charge from the turbo (assuming 10/1 CR still) 70x17.5=1225c hence the reason that proper high boost turbos have to have very low static compression ratios. For NA engine at 20c intake the cylinder temps could reach 350c during compression. Compare that 7c difference, with everything else being the same & you have a potential 127c temperature differential, work it out as a percentage ratio & it will be approximately correct regardless of the cooling/heating effects of the containment components.
Hopefully that will explain one of the reasons why gasoline engines are more sensitive to intake charge temperature.
Mowhawk2, I have one little quibble with your post. The "10/1 compression creates 17.5/1 heat increase" is complete non-sense. Temperature ratios only work, if at all, using an absolute temperature scale. Try your calculation using degrees Fahrenheit and Kelvin. Your calculation will result in completely different, and meaningless ratios (well, the ratio in Kelvin might mean something). Is 2F 100% higher than 1F? What if your starting temp is 0F?
Can't disagree with your quibble the example was pulled for the link I posted, but the effect is the same higher ambient temp will mean high compression temp. If any engine is run to the edge of pre-ignition, then a change in ambient temp may be enough to push it into pre-ignition.
As stated above modern diesels don't contain fuel during the compression phase until it's injected effectively at the last moment to ensure ignition. So they are much less sensitive to intake temps &/or compression temps.
Generally, the newest model engines equipped with direct injection systems are better on performance output and better on loss of power related with Air temperature. I tested lots of engines with multiport, single port and direct injection systems and can easy say that, DI all the way better for efficiency and the power output.
I learned that, Diesel fuel really has not exact Air fuel ratio all the time. but gasoline is always needs proper fueling under formulated dynamic or static compression ratios from 6.0:1 to 15.0:1 and more on dynamic loading from 10.0:1 over compressed air by chargers.
as brian says :
"Plenty of newer emission-controlled diesels have MAF sensors, MAP sensors, IAT sensors, wide-band O2 sensors in the exhaust, apply correction factors for the boost pressure regulation at low barometric pressure (high altitude) in order to avoid overspeeding the turbo, adjust fuel delivery in accordance with all of this, and will thus vary their torque/power output in response to barometric pressure and temperature just like a spark ignition engine will. "
newest technology, advanced engine management systems directly sets the same outputs on every temperature, and air density changes except still the gasoline worse than the diesel.
on low powered engines mechanical is above chemicals, on high horsepowered engines chemicals is above on mechanics.
