Diesel Question

[dsmith41]

This may be a stupid question but bear with me please. I am relatively new to diesels so I have been doing a lot of reading lately on how exactly they work compared to a gasoline engine.

Wikipedia has a good page on diesels but it says diesel engines produce much more heat because of the excessive compression compared to gasoline engines. If that is the case, why does everyone talk about how long it takes their trucks to heat up on those cold winter mornings? Why is it common to see diesel vehicles in general with at least partially covered grills to keep the heat in if they produce so much heat? Thanks!

 

[Cause4Alarm]

while diesels make more heat, less of it is passed to the coolant and oil and more of it to making power.....so much less heat is passed to the coolant and oil that a diesel won't get up to operating temp unless you put a load on it

 

[eljefino]

Wikipedia has been wrong before. AFAIK the thermal efficiency is slightly better on diesel than gas, so they should produce less waste heat. Since they're unthrottled diesels idle with way less fuel than gas motors so just left running they'll take forever to heat up.

Of course you're more likely to find a diesel running at 100% power for hours on end, and need more heavy duty cooling to handle that.

I think the cloth "winter fronts" also help the ancillaries like exposed fuel filters by keeping them out of the windchill slightly. The water thermostat will keep the core engine temp at the right spot. Fuel filters are great places for diesels to freeze up.

 

[Cause4Alarm]

this is also part of why diesels get great fuel mileage

 

[sciphi]

I've heard it's because diesels are better at extracting energy from the fuel than gassers. Also diesel has more heat energy than gasoline.

 

[Stewart Fan]

Another thing to consider is the engine itself. Diesels are generally more robust than a gas engine.

The Cummins 5.9 in my Ram 3500 is 1100lbs of cast iron. Takes a while to get all the iron hot.

 

[dtt004]

Diesel engines are compression-ignition engines. That means there are no spark plugs to ignite the air-fuel mixture. Think about the implications that will have on a cold morning, and a cold engine.

This is why diesel engines come with what we call glow plugs. Since it is a compression-ignition engine, the air must be at a certain temperature for reliable ignition. A glow plug assists in this.

Although the above facts are for the most part true, the most significant reason why diesel engines have a tough time starting in cold weather, on a cold engine, is because the air simply hasn't reached that temperature required for reliable ignition.

 

[dtt004]

Also, most diesel engines are unthrottled and diesel fuel has more energy per unit compared to gasoline. And since it is a 100% compression ignition engine, the compression ratio must be very high. A higher compression ratio is capable of producing far more power and far greater fuel economy than a lower compression ratio. A diesel engine is more efficient at converting the energy from combustion into torque to turn the engine, due in part to 1.) the high compression ratio and 2.) the diesel fuel.

Since most diesel engines are unthrottled, they control combustion by adjusting the fuel, rather than open or closing a throttle valve. This lessens what we call "pumping losses" or "throttling losses." An easy way to understand it is this: imagine you are driving on the freeway at a steady cruise, in a throttled engine (ex. gasoline engines). Velocity is constant and acceleration is zero. What does the throttle valve look like? It is almost closed. Air is FIGHTING its way to get into the engine simply because of pressure. Now, imagine you are accelerating hard. What does the throttle valve look like? It is wide open. Throttling losses are minimized. However, pressure still affects the fluid flow, which is why the intake tract must be designed in a way to allow a lot of air in, but when that throttle closes up, it needs to be narrow enough to increased pressure and turbulent airflow so that the air is "assisted" to overcome the throttling losses.

Forced induction engines have this problem minimized even further because the addition of a supercharger or turbocharger allows the changes due to pressure to be adjusted a bit. So there is more flexibility. However, a turbocharger/supercharger does not completely eliminate this problem because if the intake tract is not big enough under wide open throttle to allow enough air inside the engine, air will be forced back out hitting the compressor blades and counteracting its motion. The turbocharger or supercharger can be damaged as a result. However, bypass valves can be installed to eliminated this.

Back to the diesel engine. Almost all diesel engines today are turbocharged. Combined with the minimized throttling losses (there are still losses wherever we go in mechanical engineering) of an unthrottled engine, and the addition of a turbocharger, the engine can be designed in a way to favor performance while increasing fuel economy at the same time in comparison to gasoline engines.

 

[Volvo_ST1]

Originally Posted By: dtt004
This is why diesel engines come with what we call glow plugs. Since it is a compression-ignition engine, the air must be at a certain temperature for reliable ignition. A glow plug assists in this.


Not all diesel engines have glow plugs. Diesel ignites at about 750°F, a temperature that even at low temperatures is achieved simply through compression. Glow plugs make sense at far-below freezing temperatures, and even then they are not really necessary, but they make starting the engine rather a one-try-only-required experience.

 

[Shannow]

I think that the more heat reference should be simply higher temperatures of compression caused by higher compression ratios.

 

[cp3]

Is this an issue with smaller automotive diesel's, such as VWs TDI or more of an issue with HD trucks? I have always thought in the case of HD trucks or highway tractors it was more about the amount of cooling capacity required to cool the engine while being worked hard on a 100F+ day. Would that not simply take much longer to heat up at -10F?

 

[rslifkin]

^ It's a combination of the large cooling systems, heavy engine blocks, and the high thermal efficiency (and lean fuel mixtures). They don't dump a lot of heat into the coolant, except under heavy load, and they have a lot to heat up. Many also hold a lot of oil. For example, a medium duty International DT466 holds 30 quarts, IIRC.

Big engine blocks and cooling systems definitely take a while to warm up. Even my Jeep is a bit slow to warm up (3.5 - 4 miles or so to hit 200* coolant temp from a 30* start), with somewhere around 550 - 600 lbs of cast iron engine, a couple gallons of coolant and 5 quarts of oil to warm up, and it's gas. Take away a good bit of the heat going into the coolant, double the amount of metal and coolant in the system, and triple the amount of oil, and think what will happen.

 

[Thermo1223]

I inverse is also true of VW TDI's, quick to heat up when under load but also quick to loose heat at idle.

15F with 20mph wind, heat on high and you can visually watch the temp fall at idle. The name of the game is fuel efficiency and with that you produce little if any waste heat at idle under certain conditions.

Many times in the winter I would use my valet key to lock the car while running so at least I had some heat waiting for me when I got back. A 10 min trip in the store and the block would be 125F with the heat on high from a 190F operating temp.

The same is true when coasting in gear down a long hill, you can watch the temp gauge fall in the winter months.

It is one car that will almost never reach operating temp idling in the winter months,idling is more damaging than anything else.

 

[mcrn]

Originally Posted By: dtt004
Also, most diesel engines are unthrottled and diesel fuel has more energy per unit compared to gasoline. And since it is a 100% compression ignition engine, the compression ratio must be very high. A higher compression ratio is capable of producing far more power and far greater fuel economy than a lower compression ratio. A diesel engine is more efficient at converting the energy from combustion into torque to turn the engine, due in part to 1.) the high compression ratio and 2.) the diesel fuel.

Since most diesel engines are unthrottled, they control combustion by adjusting the fuel, rather than open or closing a throttle valve. This lessens what we call "pumping losses" or "throttling losses." An easy way to understand it is this: imagine you are driving on the freeway at a steady cruise, in a throttled engine (ex. gasoline engines). Velocity is constant and acceleration is zero. What does the throttle valve look like? It is almost closed. Air is FIGHTING its way to get into the engine simply because of pressure. Now, imagine you are accelerating hard. What does the throttle valve look like? It is wide open. Throttling losses are minimized. However, pressure still affects the fluid flow, which is why the intake tract must be designed in a way to allow a lot of air in, but when that throttle closes up, it needs to be narrow enough to increased pressure and turbulent airflow so that the air is "assisted" to overcome the throttling losses.

Forced induction engines have this problem minimized even further because the addition of a supercharger or turbocharger allows the changes due to pressure to be adjusted a bit. So there is more flexibility. However, a turbocharger/supercharger does not completely eliminate this problem because if the intake tract is not big enough under wide open throttle to allow enough air inside the engine, air will be forced back out hitting the compressor blades and counteracting its motion. The turbocharger or supercharger can be damaged as a result. However, bypass valves can be installed to eliminated this.

Back to the diesel engine. Almost all diesel engines today are turbocharged. Combined with the minimized throttling losses (there are still losses wherever we go in mechanical engineering) of an unthrottled engine, and the addition of a turbocharger, the engine can be designed in a way to favor performance while increasing fuel economy at the same time in comparison to gasoline engines.


Thank you! Great explanation!

 

[440Magnum]

Originally Posted By: dsmith41
This may be a stupid question but bear with me please. I am relatively new to diesels so I have been doing a lot of reading lately on how exactly they work compared to a gasoline engine.

Wikipedia has a good page on diesels but it says diesel engines produce much more heat because of the excessive compression compared to gasoline engines. If


Wikipedia is often short on facts. Diesels DO NOT "produce more heat" than gasoline engines. If you take a turbocharged diesel engine and a gasoline engine that both produce exactly the same amount of horespower at the crankshaft, the diesel will likely reject slightly LESS heat to its cooling system than the gasoline engine. It will definitely put less heat out the exhaust pipe than the gasoline engine. Turbo-diesels are more thermally efficient (convert more of the energy in the fuel into power at the crankshaft and less into heat) than gasoline engines.

Wikipedia is wrong.

 

[antonmnster]

I think Wikipedia was referring to the heat in the cylinder during the compression stroke, not the overall amount of heat normally sent to the cooling system.

The tractors at work need to be idled up to at least 1300 rpm if you're going to see any cabin heat. If they're loaded they'll heat up, but with light loads and the thermostat will never open.

There's no way a diesel will start around here without glow plugs. From the .5L 3-cyl in the Workman to the Mercedes snowcat to the New Holland tractor to the Cat loader... they will not start without heat.

 

[rpn453]

Originally Posted By: 440Magnum
Wikipedia is wrong.


I disagree. If you can find the erroneous section on the Diesel Engine page, I will correct it. But I can't find any obvious errors. I think "heat of compression" was simply misinterpreted in this situation. In fact, the page even states, in the Major Advantages section:

"They generate less waste heat in cooling and exhaust."

 

[rpn453]

Originally Posted By: dsmith41
Wikipedia has a good page on diesels but it says diesel engines produce much more heat because of the excessive compression compared to gasoline engines. If that is the case, why does everyone talk about how long it takes their trucks to heat up on those cold winter mornings? Why is it common to see diesel vehicles in general with at least partially covered grills to keep the heat in if they produce so much heat? Thanks!


The heat of compression is lost during the subsequent expansion, with only the energy from the burning of the fuel contributing heat to the overall system. In an adiabatic (perfectly sealed and insulated) piston-cylinder arrangement, the air in the cylinder will warm up as it is compressed, getting hotter as compression increases. But it will then return to its original temperature whenever it is allowed to decompress.