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Bus Discussion => Bus Topics ( click here for quick start! ) => Topic started by: HighTechRedneck on September 11, 2006, 12:31:34 PM

Title: 2 stroke vs. 4 stroke - power and efficiency
Post by: HighTechRedneck on September 11, 2006, 12:31:34 PM
A friend was helping me and he started asking me questions about diesel engines.  Most I could answer.  This one I could only speculate on so I thought I would throw it out here and see if some the diesel experts on the board could shed some light on this.

From a simplistic view it seems like a 2 stroke diesel engine would have twice the horsepower and low end torque potential of a 4 stroke with similar displacement and RPM range because all cylinders are firing on every revolution of the crankshaft as opposed to half in a 4 stroke.  Also it seems like the 4 stroke would have twice as much internal energy loss due to the additional stroke per cylinder.  On the other hand, from what I have read, it seems that the 4-strokes have more low end torque and similar or greater horsepower.

I speculated that perhaps it's because the 4 stroke has better respiration and has the full duration of the power stroke before the exhuast port opens.  But that is just a guess.  I would be interested to know the real answer.

Title: Re: 2 stroke vs. 4 stroke - power and efficiency
Post by: NCbob on September 11, 2006, 04:28:14 PM
Not wanting to come across as an "authority" on the subject...but if you look at the OTR truckers records...they'll pretty much answer most of the questions you've asked.

While I, personally, lean toward the 2 stroke Detroits...it's only because I have spent sooo many years with them and love the sound (you may read 'scream' if you'd like) but...there's no comparison as far as twist and HP that the Engineers are able to get out of the good in-line sixes (four strokes).

The experts will be along directly and offer graphs, and stats and I won't contest their offerings.  When it comes to trucking...the business of business is profit...and that's what the line haulers and independents are looking for.

When it comes to your bus...I believe it's a case of: are you willing to live with the powerplant that came with the bus  (sorta like run what ya brung) or do you have a driving desire to not only outwit the original designers, who were limited by what was available at the time, and run with the big boys?

One thing which has been proven over the years is that the more fuel you put through an engine...the shorter it's lifespan will be!

Do it your way...and you'll be the happiest person behind the wheel....in your bus!



Title: Re: 2 stroke vs. 4 stroke - power and efficiency
Post by: Dallas on September 11, 2006, 04:54:10 PM
I love this question!
Being an old "Screamin' Jimmy" guy at heart!
I have a soft spot for the venerable 2 cycle detroit engine.
They are still being built, rebuilt and used around the world today.
One of the reasons they aren't used in the trucking industry is that Detroit decided not to try to compete with the four cycle emgine when it came to emissions regulations, after all, they engine made it's debut in 1938, it was time for a change.

On a side note, the most power ful diesel engine in the world is the Wartsila-Sulzer RTA96-C turbocharged.

 Some facts on the 14 cylinder version:
   Total engine weight:    2300 tons  (The crankshaft alone weighs 300 tons.)
   Length:    89 feet
   Height:    44 feet
   Maximum power:    108,920 hp at 102 rpm
   Maximum torque:    5,608,312 lb/ft at 102rpm

It is also, (I think), one of the most efficient diesel engines in the world with 50% thermal efficiency as opposed to a normal automotive engine which is around 25% efficient.
(All these figures are taken from various places on the web, but one place with good photos is:


You might also consider that the 6-71 can put out around 238 hp with a displacement of around 426ci. or 1.8 hp per cubic inch
Or about 6.3 Litres, 38 hp per litre.
The 12 litre 4 stroke Detroit puts out around 450 hp or 35.4 hp per litre.

I also still have a soft spot for the old Cat engines too, but, hey, I'm *ussy whipped!


Title: Re: 2 stroke vs. 4 stroke - power and efficiency
Post by: TomC on September 11, 2006, 05:02:44 PM
HTR- you're partially correct.  The 2 strokers really don't have twice the effective cubic inch.  On paper, an 8V-71 has 567.45 cu in or by 2 stroke theory should equate to 1,134.90 cu in compared to a 4 stroke.  But in reality, since the exhaust valves on a 2 stroke opens about 1 inch before bottom of stroke to let out the initial high pressure exhaust gasses, the 5 inch stroke really would be considered a 4 inch usuable stroke.  So the actual displacement of the 8V-71 compared to a 4 stroke would be 908cu in.  Also, because of the opening of the exhaust valves of the 2 strokers at 4 inch with a 5 inch stroke, the torque is not as high as say a Series 60 that has a 6.3 inch stroke that uses all of it before the exhaust valve opens.  Some of the reasons the 4 strokes are more effiecient than the 2 strokes; 2 strokers need a blower that can take up to 50 hp to power just to make the engine run-although the bypass blower valve on turbos negatates that to a high degree on full throttle; because of the intake ports, there is some mixing of fresh air and exhaust gases; they can't be made to run as clean as the 4 strokers since they can have exact injection with their individual 4 stroke compared to the overlapping 2 stroke funcitons.
As to fuel effieciency, if you start with the non-turbo 2 strokes, graduate up to a turbo and aftercooled engine, and you should get 1-2mpg better.  Graduate from a turbo mechanical injection to an electronically controlled injection and add another 1-1.5mpg.  Change from a 2 stroke electronic to a 4 stroke mechanical, and the fuel mileage will be about the same.  Change from an electronic 2 stroke to a electronic 4 stroke, the electronic 4 stroke will get 1 mpg better than the 2 stroke.  So changing from a mechanical non turbo 8V-71N to a similar powered Series 50 4 cylinder 4 stroke electronic would be 3-5mpg better, or if you're getting 5mpg with the 8V-71N could get 10mpg with the Series 50.  Makes you think?  Good Luck, TomC

Title: Re: 2 stroke vs. 4 stroke - power and efficiency
Post by: TomC on September 11, 2006, 05:13:16 PM
Comment on the big Wartsila-Sulzer engine.  It has a 38" bore x 98" stroke compared to the 8V-71's 4.25" bore x 5" stroke.  What is interesting to me is piston speed comparison.  Take the big engine running at 102rpm, and at the same feet per minute translates into the 8V-71 running at just under 2,000rpm!  That is really moving, and considering Detroit rated the 8V71 at 1,800 rpm for continuous operation at 240hp, when compared to the big engine the 8V71 would have to be putting out 701hp at 1,800rpm.  Don't think the 8V-71 would last to long.  So that big engine really puts out some power for even its' huge size.  Good Luck, TomC