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Stroke-to-Bore Ratio: A Key to Engine Efficiency

Dr. Randy Herold, Engineer IV, General Atomics Aeronautical Systemsby Dr. Randy Herold
Engineer IV
General Atomics Aeronautical Systems

 
While there are many factors that contribute to an engine’s efficiency, the primary factor that needs to be considered is the engine geometry itself. Not only does the overall size of the engine matter, but the aspect ratio of the engine cylinders—defined by the stroke-to-bore ratio—also matters. To explain why, one must consider three factors: in-cylinder heat transfer, cylinder scavenging and friction.
 
Simple geometric relationships show that an engine cylinder with longer stroke-to-bore ratio will have a smaller surface area exposed to the combustion chamber gasses compared to a cylinder with shorter stroke-to-bore ratio. The smaller area leads directly to reduced in-cylinder heat transfer, increased energy transfer to the crankshaft and, therefore, higher efficiency.
 
Cylinder scavenging—a two-stroke phenomenon in which the exhaust products in the cylinder are replaced by fresh air—is also strongly affected by the stroke-to-bore ratio in a uniflow-scavenging, opposed-piston, two-stroke engine. As the stroke-to-bore ratio increases, so does the distance the fresh air has to travel between the intake ports at one end of the cylinder and the exhaust ports at the other end. This increased distance results in higher scavenging efficiency and, as a result, lower pumping work because less fresh air is lost via charge short circuiting.
 
Engine friction is affected by the stroke-to-bore ratio because of two competing effects: crankshaft bearing friction and power-cylinder friction. As the stroke-to-bore ratio decreases, the bearing friction increases because the larger piston area transfers larger forces to the crankshaft bearings. However, the corresponding shorter stroke results in decreased power-cylinder friction originating at the ring/cylinder interface.
 
At Achates Power, we have conducted extensive analyses in all three areas in order to correctly identify the optimum engine geometry that provides the best opportunity to have a highly efficient internal combustion engine. In-cylinder simulations have shown that the heat transfer increases rapidly below a stroke-to-bore ratio of about 2, engine systems simulations have shown that the pumping work increases rapidly below a stroke-to-bore ratio of about 2.2 (because of the associated decrease in scavenging efficiency), and engine friction models have shown that the crankshaft bearing and power-cylinder friction values, for the most part, cancel each other out for our opposed-piston, two-stroke engine.
 
It should be noted here that in an opposed-piston engine—where there are two pistons per cylinder working in opposite, reciprocating motion—the “stroke” results from the combined motions of the two pistons and is roughly double the distance that one of the pistons travels in half a revolution. This fact allows an opposed-piston engine to have much larger stroke-to-bore ratios than an engine with one piston per cylinder without having excessively high mean piston speeds that are detrimental to inertial loading and friction.
 
For context, below is a plot of power density versus stroke-to-bore ratio of some current four-stroke engines designed for a wide range of applications. Note that all of the engines in the chart have cylinder heads, so the stroke describes the actual piston stroke. The data in the plot show a trend in which engines that require high power density—like those in race cars—have a small stroke-to-bore ratio, and engines that require high fuel efficiency—like those in heavy-duty trucks and marine cargo ships—have a large stroke-to-bore ratio.
 
Power Density vs. Stroke-to-Bore Ratio Graph
The limiting factor in this relationship is the inertial forces origination from the piston motion. To achieve high power density, the engine must operate at a high engine speed (up to 18,000 rpm for the Formula 1 engine), which leads to high inertial forces that must be limited by using a small stroke-to-bore ratio. For applications that demand high efficiency, a long stroke-to-bore ratio is necessary and, again because of the inertial forces of the piston, requires a slower engine speed and lower power density. For the marine application that has a 2.5 m stroke, the engine speed is limited to 102 rpm.
 
In comparison, the Achates Power opposed-piston, two-stroke engine is being designed with a stroke-to-bore ratio in the range of 2.2 to 2.6. This range of stroke-to-bore ratio values allows us to create a highly efficient internal combustion engine while still having mean piston speeds comparable to engines currently available in medium- and heavy-duty applications. Any opposed-piston, two-stroke engine with a stroke-to-bore ratio below 2 will suffer from high in-cylinder heat transfer and poor scavenging, both of which act to reduce the engine’s overall efficiency.

  • Murugesh says:

    Thanks for this post. You have answered one of my important questions.
     
    I am a CFD engineer from India.

    June 25, 2012 at 4:13 am
  • philip jacobjack says:

    1. in internal combustion engine, why it that when ever a valve is bad or worn out its always the inlet valves? 2. How is air/fuel mixture delivered into the combustion chamber in si engines that uses petrol injectors

    September 5, 2012 at 2:38 am
    • admin says:

      Philip:
       
      Thank you for your questions.
       
      1. Your statement that it is always intake valves that fail is incorrect. Exhaust valves are more likely to fail from thermal loading as they get much hotter and are exposed to corrosive exhaust gases. Intake valves are more likely to fail from mechanical overloading because they typically have wider heads. But there are a variety of other failure modes that affect both intake and exhaust valves, including wear, manufacturing faults (such as heat treatment), deposits, etc. If you want to learn more, we can suggest you purchase one of the many books available from the SAE, such as “Engine Failure Analysis” by Ernst Greuter and Stefan Zima.
       
      2. Petrol (or gasoline) spark-ignited engines are generally run with a stoichiometric air/fuel mixture – that is, just enough air to burn all the injected fuel. They are run at this condition because it results in the highest emission reduction efficiencies for the three-way catalysts that are used to clean up the exhaust. At times, though, they are run with enrichment (more fuel than can be burned with the available air) to provide additional power and/or cool some of the components. The engine computer/controller adjusts both air and fuel by commanding the correct opening for the throttle in the air handling system and by commanding the correct opening duration for the fuel injectors, respectively. The engine controller uses a signal from the accelerator pedal to determine how much fuel is needed and a signal from a HEGO (heated exhaust gas oxygen concentration) sensor in the exhaust to have closed loop control over the air/fuel ratio.
       
      John Koszewnik
      Chief Technical Officer
      Achates Power

      September 10, 2012 at 9:21 am
  • Abhinav Sharma says:

    HI,
     
    Its so useful to read the information provided by you, I want to ask you that what is the real time difference between Cruising,idealing,starting and accelerating in an IC Engine in terms of A/F mixture.
     
    and through your expertise in the subject has impressed me a lot so can you provide e with some objective question and answers in IC engines that could further ake my fundamentals clear.
     
    waiting for your precious reply.

    September 11, 2012 at 10:54 am
  • Larry Fromm says:

    Thank you for the kind words. We have been fortunate in assembling a team of experts from around the world to develop our fuel-efficient and low-cost opposed-piston engine.
     
    As a small company working very hard on our own product, we are probably not the best resource for general engine questions. There are many good books and other resources on the technology – one I use is “Engineering Fundamental of the Internal Combustion Engine,” by Willard Pulkrabek.
     
    Larry Fromm
    Vice President, Business and Strategy Development
    Achates Power

    September 12, 2012 at 9:22 am
  • Aghurri says:

    Thank you for that useful article. Please answer the following questions.
     
    Is it possible to make higher rpm and less torque diesel engine by decreasing the stroke to bore ratio? Is that practicable?
     
    What the (lower) limit of stroke to bore ratio for diesel engine?
     
    Aghurri
    Indonesia

    October 21, 2012 at 3:14 pm
    • Larry Fromm says:

      It is possible to decrease the stroke-to-bore ratio, and this will allow the engine to operate at higher RPM without generating excessive piston speed and friction, but to optimize efficiency you generally want a longer stroke-to-bore ratio. Most diesel engines for light trucks and larger vehicles have a stroke-to-bore ratio above 1.0 because of the focus on efficiency, but lower stroke-to-bore ratios are possible. We have not explored the lower limits because our focus is on optimizing efficiency.
       
      Larry Fromm
      Vice President, Business and Strategy Development
      Achates Power

      October 22, 2012 at 1:56 pm
  • Vibin says:

    Dear sir,
     
    If I increase the stroke of a standard engine, in terms of power output, will the efficiency increase? Is a high speed low torque engine preferred or a low speed high torque engine? With respect to efficiency and power generation.

    May 25, 2013 at 2:15 pm
    • Dr. Gerhard Regner says:

      Vibin:
       
      The optimal set of design parameters depends on the application-specific requirements. Increasing the stroke of an engine results in some factors that increase efficiency (like a decrease in the surface area-to-volume ratio of the combustion chamber) and some factors that decrease efficiency (like increased friction from higher piston speed, if the engine speed is kept the same). So the right tradeoffs have to be balanced with the demands of the application, including power and torque requirements, package and weight constraints, cost considerations, transient operation, and drive shaft speed.
       
      Dr. Gerhard Regner
      Vice President, Performance and Emissions
      Achates Power

      May 30, 2013 at 9:39 am
  • Amol says:

    how much is the stroke bore ratio for long, superlong and ultralong stroke engines???

    June 25, 2013 at 10:14 am
  • anil says:

    Is it true that long stroke engines are much smoother than short stroke ones and that long stroke engines employ heavier flywheels to carry over dead centers effectively? Do they develop max power and torque at low speeds?

    September 14, 2013 at 6:10 am
  • jeffrey says:

    what is the common speed of the crank in rpm in 4 stroke 4 cylinder diesel engine? and its stoke displacement?

    September 26, 2013 at 9:07 am
  • john censier says:

    Hi Randy. I’m not a mechanic but understand how engines work. HUGE F1 and Ferrari fan. I was trying to find out the other day what allows an F1 engine to rev so high and why ddo Ferraris in particular have such a high ptched waling sound that sso many people like? I always envisioned F1 engines as having piston with diameters of say 1 1/2 – 2″ thuss les mass to be moving around but the other day I looked on the internet and found that the bore was roughly equivalent to that of a Chevy V8 (4″ or so) so I figured it must be a very shorrt stroke length. Also I would assume that the heads being pneumatic actuated rather than mechanical would have are a mojor factor.
    VERY quickly, so as to not take up your time, can you give me a quick understandable answer? Thanks JC

    October 9, 2013 at 5:53 am
  • Nick Hall says:

    I am buying an new C7 Corvette with the direct injection 350 2014 chevy block. I am considering boring this to 383 or 417 or 427. From reading this post I am thinking that MPG will not be substantially impacted with the increase stroke (383) or the small bore 417, or the larger bore 427. But not sure which if any to select. I am looking to increase the torque of this engine, but I do not want to give up to much mileage (MPG). So I am wondering what you think the effect will be one steady state 60MPH, flat road, no wind, 70 degree day driving? Any info or advice you can provide is wonderful. And I have never seen a better posting on these subject than what is on your site. Nick Hall

    December 7, 2013 at 10:13 am
  • Rebekah Davis says:

    3.76/3.20 is the bore and stroke ratio on the mini van I am interested in. (Nissan Quest) I want to make sure the engine is a good fit for my payload- 6 heavy people and our stuff. If I understand this correctly, at least as far as race cars go, if the bore/stroke is too long, there isn’t enough horsepower, but if is too short, it will overheat too easily ? Am I right in feeling good about the ratio on the Quest then? Any answers are appreciated. Thx.

    December 31, 2013 at 1:40 am
  • james woodhouse says:

    Dear achatespower.com
     
    What is the optimum engine size? So as an engine is built larger its friction and weight counter some power produced and too small an engine would not have the power to drive a vehicle effectively. Is there a point on a graph where the lines cross?
     
    An engineer on a course I was on stated it was 2.0l. He was not an automotive engineer though.
     
    Thank you.
     
    James Woodhouse

    January 12, 2014 at 1:56 pm
  • Anshul says:

    Awesome description sir!

    February 11, 2014 at 9:39 am
  • Kevin says:

    Hello is it possible to get more power output from my 3.0L which is currently at 90.6 bore and a 83.3 stroke to achieve more from is Naturally Aspirated development which gets its boost at around 2800 rpm also how is it that a 2.0L is able to have more torque than a 3.0L and achieve more HP?

    March 26, 2014 at 3:02 pm
  • aman says:

    no superlatives can do justice to this knowledge you have imparted. keep it up.

    April 9, 2014 at 12:33 am
  • Jot says:

    i want to ask that i m a bike modifier and i am facing some problem
    i want to ask that if i m having a 500cc single cylinder engine ,if i decrease its stroke length by shifting the pin of the connecting rod slightly down on the crank shaft then what will happen to the performance of the engine , will there be any effect of it on engines cc
    will its average and bhp increase
    please help me out

    May 21, 2014 at 7:35 am
  • mariam says:

    Hi
    I was looking on the new type of energy saving Ultra stroke ship engines and was particularly interested in the bore ratio when I came across your article. This is very informative. Can the same principle be applied to a big ship engines.

    August 19, 2014 at 2:28 am
  • Christer Lund says:

    Hello

    A really nice read but im wondering what you think about a twin two stroke engine that are square? Is that optimal or is it other enginge configurations that are better?

    Best regards Chris

    October 7, 2014 at 12:20 pm

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