educate me please

[trouttrooper]

may not be the right place to ask this, but here it is anyway. first let me say i am not trying to start any arguments, i have a basic knowledge of how diesel engines work, but not too much beyond that. one thing I've noticed you guys seem to comment frequently on is the battle of keeping diesel engines from overheating. not just dmax's either. is it the turbo aspect causing it? in my early years of owning my beast I put the big block through sme extreme situations. ....20,000 lbs going up 12% grade in 112 degrees. not once did the engine temp get anywhere near a point of concern. the allison would get a little warm, but would drop quickly once the hard part was done. the variety of GM 6.0 engines I've used and abused over the years have never had overheating concerns. why aren't they putting bigger radiators in. is it a space issue? i have my own reasons for not owning a diesel, but i know they have a rightful place in a lot of applications.

who knows, if they would make a "real" midsized suv with a baby dmax that got 40mpg and didn't cost a fortune I'd be all over it

 

[schiker]

Radiator size is a big part of it. And air through the radiator too. After that its a big combination of turbo efficiency, air flow through the intake/engine, water pump, oil cooling, emissions, transmission etc etc.

The piston in a Diesel sees more heat load than a gas piston typically. In a NA gas engine the piston is hot half the time. It is hot on power and exhaust stroke. In a diesel its hot almost 3/4 of the time all the time. Its the same or slightly hotter on power and exhaust stroke but half the compression stroke is hot too. If turbo and non intercooled the intake stroke is pretty warm too under boost (as hot as the coolant in a gas engine). So for a non intercooled turbo Diesel more heat has to be expelled through the coolant and oil compared to a NA gas engine.

 

[Will L.]

The Diesel engine is engineered to Maintain the heat for combustion. Gas, using spark plugs, are engineered to shed heat.
A diesel has a lower btu: mechanical energy ratio. So to get 1 hp from a diesel you have to burn more btu (more over all heat generated) than from a gas engine. That's why the heater in a diesel gets warm quicker than a gas engine (to the love of us anti-cold folks).

 

[schiker]

I would say Diesels are more fuel efficient and you burn less BTU of fuel per HP output. But you do have to shed more overall heat per HP. You said it better they are designed to maintain the heat. Some heat has to be dissipated (not all just excessive heat).

Both are 4 stroke cycles (internal combustion engines). It does take more energy to start a diesel as the compression stroke takes more energy to "roll over". But once running it is more efficient because of the higher compression ratio. iirc The ideal cycle the diesel is modeled under a constant pressure for power stroke and the gasoline engine a constant volume. Because the compression stroke builds so much heat the fuel only adds a little more heat relatively. Where as the gasoline cycle not near as much heat is generated on compression and all the force comes from the spark and ignition of the fuel near TDC.

As long as you don't cool the compressed air in the cylinder theoretically it should expand and give back the same energy it took to compress it. So a Diesel typically holds a bit more heat. By function the amount of time the cylinder is at elevated temperature is more for a diesel and thus it will have shed more amount of heat soak for the same amount of HP.

 

[Will L.]

Correct. Diesel has the ability to get you higher mpg, because it has more BTU per gallon than gasoline.

It takes a lot more energy at the distillery to get the gasoline then it does to get diesel. More diesel is produced by volume in the distillery than gasoline. That is the only reason why diesel is cheaper, or comparatively priced to gasoline. If you could wave a magic wand at a distillery and get whatever you want it at the same cost per gallon, diesel fuel would cease to exist.

Roughly
116,000 BTU in gasoline
128,000 BTU in diesel
1 gallon of diesel fuel will get the engine hotter than 1 gallon of gasoline just by the fuel self itself.

Modern gasoline engines run 40 to 50° hotter than the counterparts before electronic control. This has allowed lower emissions, and heigher efficiency turning more thermal energy of the fuel into mechanical energy.

Diesel engines can not handle a 40 to 50° increase in temperature without causing detonation. Once we learn how to cool everything except for the combustion chamber better in a diesel to overcome this, the same technology can be applied to a gasoline engine and increase is efficiently as well.

 

[WarWagon]

Many things affect 'cooling'. FUBAR any one thing or (FUBAR) collect them all.

HP and MPG wars. Trying to stuff a power check the cooling "system" can't cash in. GM takes the award for this. Some Gas rigs also have cooling problems like a Trailblazer SS using a 5.3L cooling system (radiator too thin and hot underhood air return path to the stack.) with a LS2 Corvette 6.0 V8 stuffed in it. Don't get me started on R134A and GM's sub standard CCOT systems. IMO GM ignored the Phoenix Proving grounds as they were sitting in a snowbank in Michigan trying to defrost their asses. I mean Heated Seats are a safety problem here if they turn on accidentally.

Airflow through the radiator stack from aerodynamics - plows, lifting the truck, front air dams, body sheet metal bends, stupid stuff like grills with 25" solid bowties blocking the radiator air... having the engine 1" behind the fan, stack restriction (too many airflow restrictive radiators). Bugs and dirt buildup reduce cooling. Hot underhood air returning to the front of the stack from poor OEM stack seal design.

Fans - delay to turn on (ECT spike while waiting for the damn Obsolete Spring Thermal fan to lockup), type electric:belt drive clutch:EV, and down to the CFM it can deliver and how fast it can deliver.

Radiator capacity and thickness.

Engine RPM. driving the compressor, oil pump, water pump and fan RPM if applicable. Not enough RPM means not enough cooling. In fact the wrong gear and lots of power in a MT truck can severely overheat the MT oil. Lugging the engine from wrong gear in automatics when pulling a grade in double overdrive aka 6th *Cough 2008 Duramax Cough*

Stack BTU Heat load. Outside air of 105 degrees has the AC condenser at 160 degrees, CAC is hotter, and whatever cooling BTU via air temp rise is left cools the engine radiator. If the CAC overheats from turbo overspeed, aka bad tune, it can blast furnace the radiator... 3 "Oil" (PS, Trans, engine oil) coolers also add heat. There is even a Fuel cooler under some trucks. Point is to have cooling the radiator must be hotter then the air flowing over it. The other radiators heat the air up a lot before the engine radiator so it's seeing 160+ degree air not the 105 degree outside air temp.

Yeah, the exhaust is part of the cooling system. So cats and DPF's are restrictions requiring more cooling elsewhere. Say the turbo that's too small trapping heat in the engine that would normally go out the exhaust.

Intake - Hot Air intake to melt snow off the air filter, but, can affect turbo outlet temps. Restrictions to the inlet same problem of heating the CAC up on some things.

Then you get down to the design of things like High output Water pumps, piston cooling oil squirts, coolant concentration, water wetter, not enough steam vents from the block/heads to the radiator, Evans, dual Tstats for cooling system flow, balanced flow to both V8 banks...

 

[MetalMaineak]

The heat thing is probably engineered into the newer engines, but I come from a time where I was trained to work on '70s era Detroits 3/53 - 12V/92TT, 250 -450v8 NA Cummins, V12 air cooled Continentals, and White turbo Multifuels They all but the White ran CRs of 22-24:1 Even my '84 Diesel Rabbit had a 21:1CR. Todays diesel engines run around 18:1 mostly to meet EPA emissions guidelines so they do run hotter. I remember those Winters in Germany where we blocked off the entire radiator and still froze to death LOL Emng temps struggled to get to 120-130 deg F

 

[THEFERMANATOR]

The heat thing is probably engineered into the newer engines, but I come from a time where I was trained to work on '70s era Detroits 3/53 - 12V/92TT, 250 -450v8 NA Cummins, V12 air cooled Continentals, and White turbo Multifuels They all but the White ran CRs of 22-24:1 Even my '84 Diesel Rabbit had a 21:1CR. Todays diesel engines run around 18:1 mostly to meet EPA emissions guidelines so they do run hotter. I remember those Winters in Germany where we blocked off the entire radiator and still froze to death LOL Emng temps struggled to get to 120-130 deg F

The biggest reason for the reduced CR is indirect injection VS direct injection. Direct injected diesels have always had CR under 20:1, most today are down in the 15-17:1 range. Indirect injected diesels still run CR in the 20:1 range and higher. But higher CR does lead to higher NOX output. So reduced CR and cooled EGR to even further reduce cylinder pressure's is what they've done