Marine Engines A to Z

engineWhat does an engine and a good cup of coffee have in common? It might enable me to convey a lot about engines in a minimum amount of space…There are several good books written about engines; but, they have the luxury of space and time from a devoted reader. Some readers tire in the details early on, not having a simple true conceptual understanding of the engine itself.…Seriously, I think if you start at the back and work forward with a few of the books that I have read, they would be more comprehensible particularly to the initiate. My intention initially is to give you enough solid information to go to a boat show, talk intelligently to the engine representative and make informed engine comparisons. When you finish scrolling this link, you should have an understanding of how marine engines work, be able to diagnose problems, the confidence to make your own repairs and the enthusiasm to dig deeper and learn more. Learning about engines is a way of thinking. Halfway through, what might appear to be a digression, is purposeful. I found, in myself, as a navel architect, when one becomes creatively involved and aware, necessity becomes the mother of invention. That’s how engine systems evolved. …That’s the focus of this link. Leading you through the logical process, it’s hopefully intended by the end of the link, that you’ll know engines A to Z. Besides, women love men who know engines. Just ask my good friend Corina, a South Carolina state insurance agent. “You always make Lovin Fun for this sailor…Doll”.

Let’s go to Starbuck’s…get a virtual cup of coffee and begin with basic principles.


An engine begins with a large block of metal, an engine block. You probably at one time or another have seen it with generally four, six or eight large holes bored into it..with maybe each hole approximating a diameter of four inches all lined up.These holes are your engine cylinders. They hold the fuel that makes your engine run.

Let’s see what they have in common with the two cups of coffee we just ordered. Imagine the coffee cup, the same size and virtual rigidity of the cylinder. Before we drink the coffee we’ll put the lid back on the cup…The lid might be conceived similar to the engine head. Now that the lid is on the cup, we’ll presume that it won’t come off. Now take a look at the bottom of the cup. We’ll suggest also that the bottom of the cup can move in the direction toward the top (up and down…reciprocal motion) The bottom now has the capacity within the walls of the cylindrical cup to act similar to the head of a piston.. We will give the cup the same rigidity, for our simple exemplification, presupposing that there is of course no leakage, breakage or bulging of the sides. Now we’ll ask Superman to join us, and squeeze the bottom of the cup toward the top….The applied force he’s using is the “mean effective pressure”….Since there is no bulging, bursting or leakage the coffee becomes compressed. How much depends upon the mean effective pressure applied. When it’s compressed eight times this is generally the compression ratio of a gasoline engine. When Superman really gets into it he compresses it 16 times this is generally the compression ratio of a diesel engine.…Since he had to use more mean effective pressure… it suggests a good reason why diesel engines are heavier than gas engines, i.e. you need more of a rigid cup. Let’s not get at this point gas and diesel engines confused…apples and oranges etc..but now we have two key engineering terms A-Mean effective pressure and B-Compression ratio..the greater the compression ratio, generally the more power the engine has…but again let’s keep oranges and apples separate.

By putting the cups back on the table and viewing them in profile, the distance the bottom traveled to the top approximates almost the height of the cup itself, less the small space occupied by the compressed coffee.… This distance is another engineering term called the stroke… Now take a look at the diameter of the cup “four inches” similar to the hole bored into the metal block …similarly we will call the cup diameter the bore “that makes sense”. The coffee itself, in the cup, is called the displacement of the cup (or cylinder). What we now have are three cups of coffee sitting on the table…your cup, my cup and Superman’s cup. We now have a virtual three cylinder engine. “How big is this engine?”…. We calculate it by the displacement, the amount of coffee in the three cups.…The bigger the cups, the more coffee, the bigger the engine… No… don’t be smart and calculate the ounces, a little math is in order to find the cubic inch displacement. OK…This is how we measure it. Lets get back to high school freshman math. Remember the Greek letter “PI”…You remember “PI” x “R2″…. we apply it to find out how much coffee we have in cubic inches…Lets say that three friends with coffee cups came over and sat down, we now have a virtual six cylinder engine…. We remember that PI = 3.14159.… Now if the bore (diameter) of the cup is 4 its radius is 2…,squared =4 X the stroke which we know as the height of the cup 4″. So we have all our essential information… 3.14159 x 4 x4 =50.264 X 6= 301 cu inch engine…Coffee may be measured in ounces but engines are measure in cubic inches or liters …This 301 cu inch engine, (divided by a constant 61), is similarly expressed as a 4.9 liter engine. “You might have wondered what that 2.0L or 5.0L on a back plate of autos were… as part of their model…now you know…it’s the liter displacement of the auto engine”

If you take a look at an engine specification sheet or several sheets to compare, it‘s surprising how much you know at this point. Lets take another engineering term that we think we know; because, our familiarity with the term starts at about five “horsepower” It’s derived from the work one horse can do in one minute, i.e. raising 33,000 pounds one foot.(Good trivia question for an eight year old) Engineers indicate horsepower by measuring the pressures of the cylinder at every stage of the revolution in a combined formula of piston area, length of stroke and the number of cylinders to arrive at a theoretical figure…The designation of an engines horsepower can be confusing do they mean Indicated ?(less hp. of an engine in descending order) is the brake hp, then shaft hp, then prop hp then effective careful in your comparisons.

Before Superman leaves let’s have him do a few more things to the coffee cup and wind up our discussion on the engineering considerations of marine power plants. Let’s have him attach (as only he can) a spoon to the bottom of the cup.…It’s now easier for him to push the bottom up…like a rod… and compress the fluid…We’ll call that the piston rod… Now we’ll drill a hole in the bottom of the vertical standing rod and insert a pencil poised sideways in the hole,…this is your crankshaft…The hole is more or less another term you’ve heard once or twice, a bearing .When Superman pushes up on the bottom of the cup and compresses the fluid, the squeezed molecules generate heat. In a gas engine a spark plug ignites the mixture and an explosion within the cylinder drives the bottom (or piston) back downward. The rods downward motion turns the pencil i.e the crankshaft mechanically in a rotary motion (let’s leave the details out). The crankshaft is coupled to another shaft called a propeller shaft which drives your boat forward.…Another device is attached,…more or less between shafts… called a transmission which enables you to simply drive the boat forward or backward. What engineering tries to do is to take all these developments and constantly ” research, construct and develop” the most efficient systems to actuate this process. We’ve been working at it for almost a 100 years now ..This is what new models of engines you see at boat shows try to do, convey their latest modifications of these principles,… their latest systems and subsystems that makes all this possible. We’ll ask Superman to do one other thing as an introduction into lesson two engine systems. The coffee lid that doesn’t come off needs two holes. We discussed the consequences of the explosion…He compressed it on the upstroke… ignited it with a spark plug and it drove the piston downward. These two motions are simply called strokes…in this case a two stroke engine.…”Now for the holes.” When the explosion occurs you wind up with a lot of smoke in the cylinder….We need a hole for the smoke to escape.… In a two stroke engine you have an exhaust port…in a four stroke engine…i.e. (it takes four strokes for the explosion stage to do it’s work)…we have an exhaust valve. The other hole is an intake valve so more fluid… coffee, gas, diesel fuel or whatever is refilling the cup to repeat the process over and over again… These valves are timed in such a way as to be shut when the compression stroke is occuring for obvious reasons i.e.(prohibiting the fuel from escaping through the holes)… So the next question raised by a five year old is… “how does the fuel get into the cup”……through a delivery system; either, through a carburetor or a fuel injector. At this point you now know the chief engineering considerations in the working of an engine. You only lack knowledge of its delivery systems…and support systems necessary to efficiently carry out this process. One last point before moving on……Remember we said that the compression ratio of a diesel may approximate 16 to 1 as opposed to a gas engines 8 to 1 compression…at 16 to 1…there is enough heat generated within the cylinder for spontaneous combustion. A spark is not required for the explosion. Consequently a diesel engine eliminates an electrical ignition system which is problematic to some in a gas system. It’s the compression ratio that requires a marine diesel to be more sturdy of construction and more costly. .

No benefit is begotten for free. …The increased compression requires a heavier block because of the increased mean effective pressure concomitant with a higher cost… The designer and engineer are always concerned with tradeoffs. Naturally the more pressures on an engine… higher compression ratios, and higher Rpm’s, the quicker an engine can wear out. So with the application of the knowledge you have just acquired you should begin to understand a spec. sheet of an engine and make informed inquiries at boat shows… or maybe just an appreciation of the logical process in the human endeavor of invention.

Einstein in his theories proved that space and time were relative. Astro physicists are trying to clarify the theories. It is known through their efforts that it is theoretically possible to travel faster than the speed of light..There is a young South American scientist in England trying to clarify the theory even more fully. What is known is that if a body can travel faster than the speed of light… you arrive at a destination before you have left……that’s why we say time is relative……We were introduced to warp drive through Star-Trek..Einstein introduced the theory, and the work continues……”It is possible”. When you approach warp drive or faster than the speed of light… time regresses…the consequence is that we will be able to go back into the past, sometime in the future…To my mind it has already been done, sometime in the future…and our recorded mysteries suggest this might be the case. The problem is however; that, we have not advanced empirically to actualize the delivery system…or even conceptually calculate mechanically how this might be accomplished…yet……but it will happen in someone’s lifetime.

This may seem off the topic of engines but not so…. We just discussed the theoretical principles of how an engine works through a coffee cup. Mechanical engineers used physics and emerging technologies over the past 100 years and devised the systems to make the process of the controlled explosion possible… simply to create motion which allows your boat to move in a desired direction. It’s this process of creative application which we call an engine’s systems, that will be presented which should allow your creative mind to identify the associated appliances of the engine, diagnose the problem and give you the confidence to fix a problem if so desired.

The question posed by the five year old how does the fuel get to the cup is the essential question at this point… through the fuel delivery system. Suprisingly it’s quite simple if you immediately identify two other necessary systems, a cooling system and a lubricating system. This is because each of these systems, although performing different functions are somewhat similar in design. A fuel system, cooling system and lubricating system all have tanks..Therefore they all require pumps to transport a fluid to an appropriate area to perform its respective function. The lubrication tank is called the oil sump..the fuel… a fuel tank and the cooling system, if raw water cooled only, the body of water it is situated in by virtue of a thru hole(I guess the ocean is a big tank)…If coupled with a fresh water system, another tank and another pump. “Think of a glass of soda” as the tank..the straw as the delivery hose and your sipping on the straw as the pump…This is the suction process……The other side of the pump, the discharge, is the delivery process. These aforementioned systems all employ this principle. Both the fuel system and the lubrication system contain the all to familiar filters..Aside from electrical problems in a marine environment, the second most common problem is contaminated fluid delivery…or why don’t I just come out and say it, water in the fuel supply.

It is not my intention at this point to isolate problems or get into diagnostics; but, to identify the required systems enabling combustion and subsequent motion. All engines require the healthy application of these three systems. …Every problem related to the immediate healthy functioning engine relates to a restriction of a pressure, oil pressure, a flow fuel or water delivery…and temperature water cooling or lubrication. “This is why you have gauges on your console it’s very important to monitor”. Many of you already know the consequences of a faulty water or fuel pump. It can be as simple as a punctured hose ( “Remember as a kid you purposefully broke the straw and tried to sip up the soda by putting you fingers over the break”)…a pump has no fingers.

The basic appliances of these respective systems, with a little practice should be recognizable on any and all engines…(except of course the tanks on the boat).

Fuel System-Gas: tank, strainer, fuel line (the straw), filters, fuel pump, screen, carburetor to cylinder.

Fuel System-Diesel: tank, strainer, fuel line, filters, fuel pump, filter, injector or injector pump and return line to tank.

Lubrication System: crankcase sump, strainer, tubing (the straw), pump, pressure regulator, engine oil cooler and return line.…Your pressure gauge should be somewhere between 30-60psi. (watch your pressure gauge on you boats console) The pressurized oil circulates through the engine via an ingenious network of tubing, drilled holes and distribution passages called galleries.

Cooling System: water, scoop, hull thru hole, seacock, hose(the straw), water pump, thermostat, engine exhaust exit.…An addition of a heat exchanger and a fresh water pump is necessary for fresh water cooling.

It would be very difficult to live in an internal combustion environment if it wasn’t for a very simple appliance which again we are acquainted with at an early age, the muffler. This is central to the Exhaust System…. Remember earlier we made two holes in the lid, one hole was to allow the escape of the exhaust fumes from the cylinder. Since there are six cylinders in our engine there’s a device that generally looks like a long square tube, generally running along the top side of the engine. This tube has six ports in it collecting the exhaust gases. It’s easily recognizable because there is usually a thick wide flexible hose attached to the central port leading aft connected to our all to familiar muffler…another hose attached to the other end connects to the exhaust port channeling the vessels fumes and raw water from the cooling system overboard. This collection tube is known as a manifold, in this case an exhaust manifold.

Since the exhaust manifold collects the exhaust from the six cylinders of our engine it would equally stand to reason that there would be an intake manifold for the fuel……and that is the case, a bit more exotic because of its function in delivering equal charges of fuel and air, it’s connected to the carburetor and delivers the charge on the intake stroke when the valve is open… On a diesel it need not be as sophisticated, it delivers only air as the fuel, if you review the above system, uses fuel injectors instead of a carburetor.

Since we are on fuel and exhaust systems this is a good time to bring up turbochargers. A semi circular appliance that normally sits on one of the top corners of the engine. It’s intake port is connected to the exhaust stream aft of the exhaust manifold… It takes the exhausted gases and puts these gasses to work turning a turbine, which subsequently forces pressurized air into the carburetor or a diesels air intake. This forced charge increases the efficiency by increasing the compression ratio over the conventional naturally aspirated system.. i.e. a fuel charge delivery to the cylinders induced through a partial vacuum system. The Turbocharger (forced system) is usually employed on larger engines and can be quite costly but look at the performance specs: Twin GM 671 naturally aspirated models on a 37foot Sportfisherman … displacing 29,000# had a rated Hp of 620. Top speed was 25mph@2500rpm, fuel rate 40gph@0.63mpg. It cruised at 20mph@2300 rpm, fuel rate 30gph or 0.67mpg. The output when the same engines were turbocharged was 720hp. Top speed 30 mph @2500rpm, 34gph, or 0.88mpg. Cruising speed 26mph @2300rpm, fuel rate 27gph, or 0.96mpg. More efficient, but can be a pricey appliance.

We started out with how an engine works and then we progressed to it’s basic support systems…fuel delivery, lubrication, cooling and exhaust systems…even how to use the exhaust to make the engine more efficient. Now we must discuss what makes these systems work; concluding, with how the whole process is initiated. We are actually working backwards, which to my mind is the natural way of approaching the subject “necessity becomes the mother of invention”. That’s how all these systems evolved in their present form.

So what makes the pumps work and the valves open and close at the correct time. Primarily belts and gears, inventively connected to the turning crankshaft which we irreverently exemplified as a pencil. Actually the crankshaft is an extraordinary sturdy and precision built part of the marine engine. Its job is to connect to each of the six piston rods in such a way as to convert their up and down reciprocal motion, to a rotary motion. Aptly named at the front of the engine connected to the turning crankshaft is the power take off wheel. Solidly connected to the shaft this wheel of course revolves with the shaft.…By design, situated near this wheel, connected to the block itself, you will find the alternator, raw water pump, and fresh water pump.…These are what are called mechanically activated appliances. They are all connected in one configuration or another by belts to the power take off. In short as soon as the crankshaft starts to turn…these appliances are up and running powered by virtue of the belts.…That’s why when a belt breaks or slips on the raw water pump for instance…it’s quite serious and the engine must be turned off immediately because the aforementioned flow would be restricted, and the engine temperature would rapidly overheat. When you go the the boat shows, look at all the engines, pick out all these appliances, easily recognizable because of their belts. Ask questions and get spec sheets. At this point you should have a pretty good idea of engine architecture and how they work. (The proper tension of a belt is about a ¼ inch deflection).

Remember in the beginning we talked about the two holes made in the lid of the coffee cup. One hole allowed entry of the fluid, the other the exit of the exhaust gasses. We also said that at a precise moment when the cup bottom or (piston head) was in the compression stage that the cylinder head closed off the holes… The question is therefore “what opens and closes these valves” (keep in mind that we are not only talking about one cylinder but six) ? A device called the camshaft……not only does it regulate the timing of the attached valves; but, it also is designed on gas engines to drive the ignition distributor, fuel pump and oil pump. This is all done through gearing. If you want to score points with your wife or girl at the boat show and get a wow from the crowd casually ask the engine rep. whether the camshaft is connected to the crankshaft by gear or timing chain (looks like a bicycle chain)…then give out a deliberative look to the guy standing next to you. “It’s six or one”… but at any rate whatever the system, the ratio must be two to one…That means that the camshaft gear, is twice the size of the revolving crankshaft gear, meaning it will run at one half the speed of the crankshaft (remember the term big & slow). This application makes possible the correct timing of the valves. This is done by converting the rotary motion of the camshaft which is turning by virtue of its gearing to the crankshaft, into a reciprocal motion… i.e. the valve heads move up and down”… Remember they open and close the cylinders off. Connected to the cam shaft is a cam lobe. It’s these lobes which make possible the correct timing. Connected to the lobe is the valve lifter connected to this is a push rod, rocker arm, spring and lastly the valve itself. This is what’s called the valve train for an overhead valve engine. Connected to this multi-duty cam shaft is an ingenious cross shaft turned by spiral gears which actuate the aforementioned fuel and oil pumps and the distributor (part of the electrical system of a gas engine.) The important point is that all these functions are a result of the rotary action of the crankshaft resulting from the explosion.

Although there are many differences between the diesel and the gas engine the most noticeable is the electrical system. Unlike the diesel the gas engine needs an ignition source. What ignites the compressed fuel charge is the spark generated by the excessive current. This is how it works. Your battery is connected to your ignition switch……Turn the key the 12V current is now sent to the ignition coil which converts this low voltage to app. 24,000V. The rotation of the distributors breaker cam (we talked about the cross shaft etc.) moves the breaker arm through the rubbing block and opens and closes breaker points. This is connected to the ignition coil which provides the current to the spark plug wires for each cylinder igniting the fuel. It’s interesting to see that the camshaft is not only responsible for valve timing; but, by virtue of it’s cross shaft spark plug timing. (The old watchmakers were masters of this form of technology using gears, shafts, springs for inter-related events).

Just before this occurrence however; whether it be gas or diesel, another event must take place. The beginning of the combustion process is initiated by the starting motor which actuates the larger flywheel turning the crank and setting the chain of events that we just discussed in motion. The ever familiar battery changes chemical energy into electrical energy. When the key is turned the current instantaneously goes to a solenoid relay then to a starter solenoid (a switch) that connects the small rotating starting gear to the large heavy flywheel. The teeth of the small wheel mesh with the teeth of the large wheel. As soon as the crank begins to turn, the starting wheel immediately disengages, and the chain of events begins. In review, the cam shaft opens the intake valve, allowing the fuel into the cylinder, the valves are timed to close as the piston begins to compress the fuel, the spark ignites the explosion (gas engine), the exhaust valve opens..the hot engine is continually cooled by the water cooling system, the lubrication system diminishes friction in the moving parts…all commencing when the key was turned and an electrical current from the battery turned a switch, activating a small starting wheel which meshed with a larger wheel connected to a pencil…(I mean the crankshaft.) When the crankshaft turned, the power take off wheel turned, activating the alternator which began to recharge the battery. The raw water pump and the fresh water pump activated by their familiar belts at the front of the engine, are enabling water circulation to cool the engine … In our example above Re: turbocharges our GM-671 revolved over 2500 times a minute…That’s engines A to Z

At this point, hopefully, you might have a grasp of how an engine operates, its basic principles of engineering, its systems, and the methodological capacity to know how to diagnose problems. You are limited by your lack of mechanical detail or “mechanical precision” only…For instance does your engine use hydraulic or mechanical valve lifters? Now that you know what it is and what it does, finding out should be simple. The rest may come easy to you, limited by your enthusiasm on the subject. You can speak the language, now increase your vocabulary.

I hope I haven’t offended Starbucks, Superman, Einstein, Trekies or anyone’s intelligence for that matter, it allowed me to conceptually cover a lot of territory quickly. I’ve only been back from sea a week. The Atlantic and the North Sea can present a few challenges in January and February… almost as much as creating this link on this marvelous invention the computer.

Leave a Reply

Your email address will not be published. Required fields are marked *