Why do some diesels require glow plugs and others don't?

[LABanker]

I'm not a mechanic but do tinker with about everything on my place thats paid for. I have a 1966 MF135 Perkins diesel. Tempature here in Louisiana runs from a low of about 15 degrees to a high of about 104. Does not matter. Touch the key and that puppy fires up almost before you can get off the start position. No glow plugs here. Have to admit though, it lives in my garage and my truck is outside in the weather. Kind of fond of it you might say.

 

[Jasper661]

I did some part time farming in Alaska, we grew 600 acres of potatoes and another 600 acres of hay. I bought a brand new Allis Chalmers 4WD diesel tractor for the farm back in 1981, that tractor had an ether start system, we had two other diesel tractors and old Leyland and a new John Deere. When it got cold in the Matanuska velley the Allis would always start.

There are lots of horror stories about using starting fluid to start a diesel engine, I am not sure what that is all about, I suspect it may be a function of using it with glow plugs.

My old 7L Ford F250 started great in cold weather, as long as the glow plugs were working, once they all burned out a a bit of "Pyroil", would get it going until I could replace the glow plugs.

My last John Deere was a 4020, that little tractor would start any time. I currently have a Jinma, I bought used and it starts great when it is 60+ deg., with out using the glow plugs, but colder than that and it is very hard to start.

 

[mo1]

Quite an active thread and numerous opinions being offered.

Have been a long time since I did any reading on diesel designs. Combustion chamber design is the big factor on ease of starting. At the time there were 5 distinct combustion chamber designs in use and each had its own operating characteristics. Pre-combustion chambers were new and was the answer for smaller high speed diesels. Many diesels have their combustion chamber built in to the top of the piston and not in the head.

The ones with the combustion chambers built into the piston are direct injection while the ones with the chamber in the head and relatively flat pistons (like a gasser) are the indirect injection ones.

MF has received a fair amount of comments and how easy they start and without the use of glow plugs. That is one of the characteristics of the combustion chamber design used in those engines and their CR was one of the lower ones around. The engine in my 165 was 15:1 quite low for a diesel and not that much higher than found in muscle cars of the day.

My current Yanmar has glow plugs but rarely need to use them until temps get down in to the 30's. Don't know which combustion chamber design it uses.

Do a search for "diesel combustion chamber design" and you'll get a number of hits.

My little MF (a 1030, so really a Toyosha) isn't the easiest to start without glow plugs. It's an IDI with 23:1 compression and glow plugs. It can be 100 degrees outside and it will still make a few puffs of smoke and take about 5-10 cranks to start if you don't use the glow plugs. It doesn't start at all without the glow plugs at much less than room temp. However if I burn the glow plugs for 10 seconds, it fires up in half a crank with zero smoke even down into the 40s. Repeat starts on a warm engine are half a crank with zero smoke as well. The glow plug system is apparently very unusual as it's totally manual- turn the key the other direction from start against a springed detent and it burns the glow plugs. Pretty well every other IDI tractor and every IDI truck I've seen uses a timer relay unless the owner put in his own manual pushbutton relay. (That's a really popular modification on IH's 6.9/7.3 IDI V8 diesels in 80s and early 90s Ford pickups as the timer relay likes to take a dump after a while.)

The worst diesel engine I've tried to start has to have been the 329 six-cylinder diesel in my Dad's old JD 4400 combine. IIRC it was a DI engine and it certainly had no glow plugs as it had the ether injection button on it. It would take a bunch of cranks to fire and made Cheech and Chong quantities of smoke. It would turn the air in our 40x60 shed opaque when it started up, even with all three side doors open. To be fair, it smoked only slightly less when it ran.

The combustion chamber type (i.e. Direct Injection (DI) vs. Indirect Injection (IDI)) is a design decision that is made at the beginning of the design process. This decision is based on many things, and not just starting. DI engines can have larger injectors, and the injectors spray straight down (in the center of the piston). The IDI engines have more complicated cylinder heads (due to precombustion chamber) and limit injector size.

DI engines used to be MUCH louder than IDI engines. This is because until the late 90's diesel injection pumps were purely mechanical devices. With mechanical injection pumps, when the injection pump would squirt the fuel, the all of the fuel would instantly ignite & thus the loud BANG (remember how loud the old diesel over the road trucks used to be?). With a IDI engine, the fuel pre-combusts in the pre-combustion chamber (and thus the reason it is called indirect injection), then this burning fuel continues to spray out of the pre-combustion chamber to the main combustion chamber (i.e. the cylinder). This results in a more progressive burn instead of a big bang.

IDIs were far more popular in light and medium-duty trucks compared to DIs up until the mid-1990s. Cummins diesels were DI, IH/Navistar (Ford) units were IDI up until the T444E/7.3 Powerstroke debuted in 1995 and GM's units were IDI up until they brought out the Isuzu DMAX units in '99. The IDIs were more popular than DIs because they were less expensive, less complicated, quieter, started better, and were more powerful in the pre-turbo days. You could get away with a pretty simple and fairly low-pressure injection pump and injectors in an IDI vs. a DI engine as you could just piddle out fuel into a precup in an IDI vs. having to create a nice atomized pattern in a DI. IDIs also were able to use much higher compression ratios than DI diesels as they didn't have to put a bathtub in the piston for their combustion chamber. Most DI diesels are in the 16-18:1 range while IDIs are generally in the 21-23:1 range. This gave IDIs a power advantage in the pre-turbo days. Turbos put a nail in the coffin of IDIs as DIs' injection systems could deliver a ton more fuel and still run well. The cylinder heads also tended to handle a lot more boost as well. Those IH IDIs could handle about 10-15 pounds of boost with head studs while a Cummins 6BT can handle many times that much.

Modern diesels with electronically controlled injection can control the big bang by modulating the fuel volume & pressure in a way that mechanical injection systems could not. Catapillar started this w/ their HEUI system, International Harvester used 'split-shot' injectors, and then came along common rail, which is most common today. I'm not sure if this is the most common on tractors, but it certainly is on pickups, marine/industrial engines, etc.

The Tier 4 regulations largely made tractor diesels become common-rail DI designs, even many of the little ones. The reason common-rail DI diesels are quieter than an old mechanically-injected DI diesel like a John Deere 3029 or 4045 is because they inject the fuel in several pulses rather than one big one. The fuel injection pressure in a common rail diesel is 5-10 times that of a mechanically-injected DI diesel.

Neither a DI or IDI engine will start in cold weather without some type of assist. Both DI and IDI can utilize glow plugs or intake grid heaters for this assist.

Some older diesels used ether as an assist, such as that old JD 4400 combine. You can guarantee that any engine with an OEM ether system does NOT have any sort of thermal start assist system as putting ether into an engine with glow plugs or a grid heater is an explosion waiting to happen. Manuals for equipment with a grid heater or glow plugs have dire warnings to not use ether. There are many videos on YouTube of people putting ether into the intake of trucks with automatically-controlled glow plugs and blowing up their intake or a cylinder head.

 

[Gary Fowler]

May have been discussed already as I didn't read every post. My Kubota B26 needs a few seconds of glow plug to start even if it is 85F, otherwise it turns and turns and turns before starting with a belch of black smoke. It starts in about the same amount of time by just applying glow plugs for 5 seconds then it starts in about 1 second. MY Kubota RTV starts without glow plugs if it is above 40F with just a touch of the key but as with most diesels, a short cycle of the glow plugs makes it fire on first turn of the motor.
MY LS P7010 has no glow plugs, starts easily on the coldest day I have ever used it and has no ether injection system plus it only uses one 12 volt battery. I suppose gone are the days of having 200 pounds of battery to start a tractor engine although I see diesel pick up trucks still use 2-3 batteries.

 

[SPYDERLK]

May have been discussed already as I didn't read every post. My Kubota B26 needs a few seconds of glow plug to start even if it is 85F, otherwise it turns and turns and turns before starting with a belch of black smoke. It starts in about the same amount of time by just applying glow plugs for 5 seconds then it starts in about 1 second. MY Kubota RTV starts without glow plugs if it is above 40F with just a touch of the key but as with most diesels, a short cycle of the glow plugs makes it fire on first turn of the motor.
MY LS P7010 has no glow plugs, starts easily on the coldest day I have ever used it and has no ether injection system plus it only uses one 12 volt battery. I suppose gone are the days of having 200 pounds of battery to start a tractor engine although I see diesel pick up trucks still use 2-3 batteries.

Your LS has an intake air heater doesnt it? My Mahindra 7520 has that instead of glow plugs. It acts automatically with a turn of the key.
larry

 

[Stanley75]

I've found it quite interesting to read all the comments here. Following the tractors and engines for most of my life and working with gas as well as diesels.

I've worked with pony motors to start diesel engines as well as starting on gas to warm up before getting diesel fuel to start. There are some brands that I haven't had a lot of exposure with but not to many. I haven't missed to many tractor or lawn mower engines gas or diesel! I do live in an area that it gets cold in, we have seen below -20 for weeks in a row so we know what different engines start like in the cold as well as the warmth.

I've found that some start well in the cold and some don't start well even before getting to the freezing mark or until the block heater is plugged in no matter what when cold.

I like the different designs of the engines because then you might not have bench marks to know what is good. Speaking of good what is good for you, might not even start here in January or February.

Sometimes there are exceptions or reasons as there is only one that is that way! Might just be pump timing or might be more. This might include lack of compression!! We get quite technical in our diesels even setting valve protrusions when doing valve jobs to maintain factory specs on compression for our customers.

To get a diesel to run you need heat, this is caused from compressing the fuel, might it not be easier to heat a smaller area such as a pre-cup vs the whole cylinder?

There are reasons as to why things are built in the way they are, from the cost, it is more expensive to use glow plugs and have pre-cup injectors over a direct injection head.
I've also found it hard to find an engine start better in cold weather then an indirect injection system with glow plugs or one with a preheat heat system. Some might still start but you might have DEC pulling into your yard from the air polution created! This we often do for customers that perfer some of the other brands that we know have issues!

I've seen a lot of tractors that farmers park during the winter because they DON'T start in the winter when the temps go below freezing! I've also seen them switch brands because of it in a size where they need to start in cold weather.

With winter coming I know what tractors we can start for customers with out a cord going to them and I know which ones I won't. Thats for both new and used!!!!

I've seen a lot of love in this post for what people have as well as a bit of exaggeration and lack of knowledge from good comparisons.

Liquids don't compress thus fuel doesn't compress. Air in the other hand will compress. I think we all have a little lack of knowledge on this subject.

 

[Gary Fowler]

Your LS has an intake air heater doesnt it? My Mahindra 7520 has that instead of glow plugs. It acts automatically with a turn of the key.
larry

I have no idea on that for the LS, there is no indication of a heater on the instrument panel. It also doesn't require waiting a few seconds after turning on the key. Just turn the key to start and it fires right up, no waiting for something to heat up. My Kubota B26 on the other hand always requires about a 5-10 second glow plug wait before it will start. My Kubota RTV900 starts a little easier without glow plugs as long as it is above 40F or so. Even on coldest days, it only takes 5 seconds or so of glow plug use for it to start.

I think it all must have to do with the compression ratio of the engine. High compression ratio engine starts when colder without outside heat being applied.

 

[Gary Fowler]

Your LS has an intake air heater doesnt it? My Mahindra 7520 has that instead of glow plugs. It acts automatically with a turn of the key.
larry

I have no idea on that for the LS, there is no indication of a heater on the instrument panel. It also doesn't require waiting a few seconds after turning on the key. Just turn the key to start and it fires right up, no waiting for something to heat up. My Kubota B26 on the other hand always requires about a 5-10 second glow plug wait before it will start. My Kubota RTV900 starts a little easier without glow plugs as long as it is above 40F or so. Even on coldest days, it only takes 5 seconds or so of glow plug use for it to start.

I think it all must have to do with the compression ratio of the engine. High compression ratio engine starts when colder without outside heat being applied.

 

[Sawyer Rob]

The title pretty much states my question.

What makes the difference on why a diesel engine requires glow plugs or not?

I have an old JD2240 which does not have glow plugs. I've been reading about start times on machines with glow plugs and some talk about taking 5- 10 seconds to start with the glow plugs. I have no heater and no glow plugs and even at 20 degrees last week it only spun 5-6 times before firing (about 3-4 seconds).

Just curious as to what makes mine start so easy without plugs when others don't.

Thanks

I didn't read all the answers, but in MY mind the answer is fairly simple.

ASSUMEING both diesels are in good shape, with good compression, for cold starts,

Direct Injection = faster starts without glow plugs

Indirect direction = needs glow plugs...

I have plenty of both and this IS the way it always works out.

I HATE GLOW PLUGS!!

Years ago, even the small Kubota's were DI and they started instantly, with out glow plugs!!

ALL of my real tractors start WITH OUT having to glow any plugs, they are all DI!

THANK the EPA for glow plugs, that's why the mfg's went to them...I remember when it all got started down that path and I didn't like it any more then than I do now!

Did I mention that I HATE glow plugs??

SR

BTW, the "grid heater" in the Cummins, is there more to keep the cold smoke away than it is to get the DI Cummins started! I have one of those too...

 

[Todd727]

To get a diesel to run you need heat, this is caused from compressing the fuel, might it not be easier to heat a smaller area such as a pre-cup vs the whole cylinder

The heat comes from compressing the air. Fuel has a cooling effect and liquids aren't compressible. On older tractors that the throttle also cut off fuel, you can get a quicker start by cranking with the fuel off and then adding fuel after it's spinning.