View Full Version : 4-stroke Diesel question

07-15-2006, 06:48 PM
This may be more appropriate on an automotive forum, but what the hell. This is about the only forum I'm active on for the time being.

In my pondering today during my lonely 12 hour work shift I wondered. Since Diesels ignite their fuel by high compression, how do they not light it during the compression stroke? I would think to myself that by that way of thinking it would be like a 2 stroke? Sorry, its been bothering my mind. I hate when I can't figure something out that should be obvious. I mean especially to a mechanically minded fellow like myself.

Kaleun1961, I do believe you used to be a trucker if I remember correctly?

07-15-2006, 07:53 PM
i always thought diesels were 2 strokes myself. i know it does light off at the top of the compression stroke , but thats about all i know , when something breaks at work , im quick to call the office and say "send a mechanic" , but as far as 2 stroke diesels, i know the 12-71's we have downstairs are 2 strokes , and that engines that are in locomotives and the bigger boats in the river like the EMD's are 2 stroke also. boy im a lot of help. http://forums.ubi.com/images/smilies/35.gif

07-15-2006, 09:11 PM
Yes, well thanks a pant load. LOL, just kidding. http://forums.ubi.com/images/smilies/16x16_smiley-tongue.gif

As far as I know all modern highway trucks are 4-stroke diesel. For all the same reasons they don't use 2-stroke engines in cars. 2 stroke makes more pollution, is less reliable, doesn't make much power in the low rev range...etc.

07-15-2006, 09:25 PM
I'm not a mechanic but operated 18-wheelers for 20 years. I've always thought they were four stroke engines, just like gasoline engines. The first stroke is the intake of air. Second stroke is compression during which the diesel is injected into the cylinder. At the top of the second stroke the compressed diesel fuel detonates which initiates the power stroke. After the power stroke is the exhaust stroke which expels the combustion gases. Four strokes, or cycles in all.

07-15-2006, 09:45 PM
Yes, but the Diesel would be injected into the cylinder on the intake stroke while the intake valve is open. Then in a 4-stroke engine, both valves close and the piston goes upward compressing the air/fuel mixture.

I think I just figured it out after more pondering.

The compression stroke DOES ignite the fuel as the piston reaches the top of the cylinder. This sends the piston back downwards on the combustion stroke. Not so different than a car just with the absense of a spark plug. I think that threw me off.

Case closed, I don't know why it took me so long to figure that out. I can sleep now.

God I need a girlfriend or something, lol.

07-15-2006, 09:56 PM
The compression ratio in the big truck diesels is incredible. If memory serves me well, it is about 22:1! In other words, 22 litres of air are compressed into the cubic volume of 1 litre. Packs quite a punch, which is why diesels have displaced gasoline engines from the heavy truck market. Now we are seeing small diesels replacing gasoline engines in the small truck market where power is concerned. They are more expensive engines, but last far longer than a comparable gasoline engine and are thus a better value if you plan on driving a lot of miles.

07-15-2006, 11:07 PM
thats what confuses me , it operates like a 4 stroke , but theyre listed as a 2 stroke. hey Kaleun what kinda diesel did you have in your truck and what did you prefer ? i like the Cats more , because i sleep better when theyre running ,plus they dont break down as much at least not the marine engines but as far as tough , i think Detroits are nearly indestructable.

07-16-2006, 07:48 AM
Well the original question is how do they not light during the compression stroke.

The answer is they do, err at least at the end of it.

As you compress the mixture its temp goes up so that near or at the end of the stroke it ignites.

Where are thet listed as two stroke?

07-16-2006, 11:21 AM
i found this while looking at info on the EMD's they put in trains , theyre pretty much the same thing they put in the bigger boats.

The hybrid diesel locomotive is an incredible display of power and ingenuity. It combines some great mechanical technology, including a huge, 12-cylinder, two-stroke diesel engine, with some heavy duty electric motors and generators, throwing in a little bit of computer technology for good measure.

, most all of the bigger diesels like in ships are 2 stroke , and i know for a fact that the pair of green screamers we have downstairs are 2 strokes , and theyre only 12-71 detroits.

07-16-2006, 11:30 AM
When I first started my trucking career, in 1982, I went to work for the same firm for which my father worked, Loblaws. We were then using Ford Louisvilles with Detroit engines. Later we switched over to Louisvilles with Cat engines. The Detroits seemed indestructible but were noisy and leaked oil. The Cats were nice to operate, good torque at lower RPM's but could be difficult to start in the winter.

The next truck I drove was a Volvo, but it was rather gutless; the company had used the computer to restrict the engine for fuel efficiency. After that I drove Macks, which I found to be a good truck. They had the Mack engine in them which I thought was a nice, punchy motor. Last truck I drove before I got hurt was a new Volvo, which was a dream to drive. If I was able to get back to work, that would be my truck of choice. Nice turning radius and convenience features. Took some getting used to, figuring out those electronic controls.

As I did day runs, out and back the same day we had no need for sleeper berths. I'd rather drive a truck without a berth, makes for better maneuverability, sort of like the difference between the VII U-boat and the IXC. http://forums.ubi.com/groupee_common/emoticons/icon_wink.gif

07-16-2006, 12:10 PM
i never ran a boat with a volvo yet , its mosly detroit and cat's out here , i did run one with a mitsubishi and that was one of the quietest boats ive ever been on , the power was ok , but it was only a 2000 horsepower boat , i prefer detroits when i have to do alot of shifting , because its almost like instant power , the cats tend to take a minute to build up , but ooh when they do its nothing nice. EMD's are just sweet , all you hear is that whine from the turbos. ill post some pics if anyone cares to see some of them.

8 cylinder EMD

1000 horsepower Mitsubishi


THe Green Screamer itself , a 12-71 detroit


07-16-2006, 03:13 PM
This is it simplified.
Suck, bang, push, blow. A four stroke cycle. It will only ignite on the compression because that is the only time the fuel/air mix is put into the cylinder, and is the only time all of the valves are closed - thus allowing compression. The injectors work off the cam shaft, all in timing with the cylinder cycle. It would be impossible for any fuel mix to get in at any other time unless the injector is stuffed and leaks

07-16-2006, 06:27 PM
Picture of Kaleun1961

Posted Sun July 16 2006 10:30

What a coincidence, my best buddy works at the Loblaws warehouse in Cambridge. He drives a "jigger" as they call him. For those who don't know thats like a fork lift that can't lift more than maybe a foot off the ground. He managed to beat the plant work load record in a day, twice over. Which is pretty good, thats a damn big place and I can only imagine how many people must work there.

I'm into my machining trade and what not but the way he's moving up I sometimes wonder if I'm in the wrong line of work.

07-16-2006, 07:15 PM
Quote hueywolf:
This is it simplified.
Suck, bang, push, blow. A four stroke cycle
Well put! http://forums.ubi.com/images/smilies/11.gif

My heart says Detriot 6V53 GM (aural sex) 200 to 320 Hp http://forums.ubi.com/images/smilies/partyhat.gif

My head says Volvo D16 (civilised) 16 litres/610 Hp http://forums.ubi.com/images/smilies/25.gif

My respect is for Mack E7 (honest and keeps on giving all the time) 427 - 435 Hp http://forums.ubi.com/images/smilies/11.gif

My dislike is for Detroit Series 60 (overpriced and noisy) Any Hp http://forums.ubi.com/images/smilies/disagree.gif

I drive trucks and been around them for 23 years. http://forums.ubi.com/images/smilies/53.gif

Rule of thumb:

if it's small and light a petrol engines allright...
but if its big and heavy a diesel stands ready!

07-16-2006, 09:27 PM
I've always been in love with the paxman V-16 supercharged, like they had on the "old" patrollies. Kick on forever

08-17-2006, 05:34 PM
Diesel engines can be two-stroke as well as four-stroke. During the intake cycle the cylinders are filled with pure air. The fuel is injected under very high pressure into the cylinders near the top of the compression stroke when the air temperature is sufficient to ignite it.
The largest engines in the world are marine two-stroke turbocharged diesels. A typical engine size is 800mm bore and 2592mm stroke. The largest engines develop some 5,000 hp per cylinder.

08-17-2006, 05:56 PM
The Perkins CV12 Diesel used on many tanks is a 4 stroke .

08-17-2006, 10:15 PM
well im working underground with the scoups at folcanbridge and we have cats and detroits. all i have to say they all do the job rigth but everyone has there prefrence me i like them all exepte volvo there all gutless because of fuel efficency
sorry if i upset anyone bey saying there gutless this is by experience