What is reciprocating mass?

What is reciprocating mass and how does it have an effect on engine performance. Do V8s have lower reciprocating mass of V10s? Ideally, are carmakers aiming for lower reciprocating masses of engines? If we have a lower or higher reciprocating mass, what does that do to the engine?

A reciprocating mass is a weighted object that spins or moves.

In an engine, that would be the crankshaft, connecting rods, pistons, camchain or belt, camshaft(s), valvetrain, and flywheel. Every single one of those parts move as the engine runs.

Each object has a different frequency that it puts out in vibration.
Some objects spin fully around, like the crankshaft, camshaft(s), flywheel, and camchain/belt, while others just go up and down, like the pistons, and valves, or go in oblong/elliptical patterns, like the connecting rods.

Ideally, an engine with less cylinders is going to have lower amounts of reciprocating mass than an engine with more cylinders, but this is not always the case. I would say that an engine from a V-8 indy car, would have less reciprocating mass than most of the V-6 family sedans on the market, due to light weight special metals and alloys used in their construction, which contributes to their extremely expensive nature.

Less reciprocating mass allows an engine to have a much quicker throttle response, and will typically seem to be livelier, and rev up faster. More reciprocating mass makes the engine feel slower, and less responsive to throttle inputs.

Typically, car companies have to balance out how light they are willing to go with engine components, due to cost, application (is it a sports car, or a work truck engine?), and desired feel (you’re not going to want an Indy car throttle response in your family sedan that your 15 year old is learning to drive in, in most cases).

Hopefully this answers your questions for you.


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Reciprocating mass is that which changes direction, like a piston or connecting rod. Then there’s rotating mass, like the crankshaft or flywheel. Minimizing reciprocating mass improves engine balance.

You can try out reciprocating mass and engine balance with several engine configurations yourself: hold your fists out in front of you and move them rapidly up and down, together. That’s like a parallel two-cylinder engine with a ‘360’ degree crank, like old British motorcycles. Now do the same thing, but have one go down while the other goes up. That’s the same, except it’s an engine with a ‘180’ degree crank, like Japanese twins such as Honda used. Notice how your arms balance each other out more with that one? Finally, move your arms horizontally side to side. That’s like an opposed twin, similar to the BMW twins. Notice how smooth that is? Same reciprocating mass in each case (your two fists/arms), but different balance.

You’ve already gotten an excellent definition (thank you once again Texases). But I wanted to elaborate on second and third parts of your question.

Yes, carmakers want to lower reciprocating masses. A mass in motion tends te remain in motion, a mass at rest tends to remain at rest. It takes energy to alter either of those conditions. It takes energy to begin the masses moving, and it takes energy to change their direction, as in the pistons going up and down. The bigger the masses, the more energy it takes.

In a regular internal combustion engine, 100% of the energy comes from the gasoline. Energy used to continually change the direction of the reciprocating masses (the pistons, the rocker arms, etc) is energy that’s not used to move the car forward, and it takes away form gas mileage and lower emissions.

So, reducing reciprocating masses in any given engine improves gas mileage and emissions levels. And allows more of the gas energy to be used to move the car forward, improving performance.

Reducing reciprocating masses also helps make the engine smoother. Masses constantly moving back and forth cause vibration. The lower the reciprocating masses for a given engine, the less the vibration. That reduces wear as well.

There is an engine called a Wenkle Rotary Engine, that Mazda uses in one of its cars, currently the RX8. Its parts spin in elliptical circles rather than reciprocate. That engine is very smooth and can spin at higher RPMs. The main problem with it is that because of the complex geometries in its cylinders it’s difficult to seal and almost impossible to successfully rebuild.

Electric motors are far smoother than gas engines for the same basic reason…they have no reciprocating parts.

Cool thanks! In what ways can reciprocating mass be reduced? (other than simply reducing the weight of the components)

Turbine Engine, AKA a Jet Engine.

Eliminate them. Ford stuffed a Mustang with a dual cam 5.0L V8 instead of a pushrod. 4 cylinders have less overall mass than a V6. A turbocharged 4 making similar power to V6 seems like the wave of the near future.

In racing engines, reciprocating mass is kept as low as possible to allow very high RPM to be achieved. High RPM means more horsepower…In multi-cylinder engines, the reciprocating mass of each piston, pin and rod can be kept very low which allows very high RPM. Can you imagine a 2.5 liter V-12?? Think 800 HP @ 15,000-18,000 RPM…

Reciprocating engines are trying to tear themselves apart. Mass increases with the cube of the RPM…So if you start with a very low mass and a very short stroke which reduces piston speed, then very high RPM (and thus horsepower) numbers can be achieved without blowing up the engine…

At the other end of the spectrum are the old air-cooled reciprocating piston aircraft engines. These huge engines ran at low RPM and depended on high-octane fuel (140 octane in some cases) and turbo-supercharging to produce serious power…

Mass increases with the cube of the RPM…

Care to elaborate on this breakthrough in physics?

At the other end of the spectrum are the old air-cooled reciprocating piston aircraft engines. These huge engines ran at low RPM and depended on high-octane fuel (140 octane in some cases) and turbo-supercharging to produce serious power…

Not to mention the hugeness of the engines.

Other techniques used to reduce reciprocating masses are
*) smaller pistons and connecting rods. The lesser amount of material reduces mass.
*) overhead cams. Pushrods were reciprocating masses.
*) better materials and manufacturing technologies. Modern reverse-gravity castings are far higher quality than old poured castings, with fewer inclusion and exclusions. Less material is required to make a strong part. Much less material.
*) better alloys.
*) far more consistant and accurate manufacturing technologies. These create better balanced engines. The better balanced the engine is off the production floor, the less vibration it has to endure and the less strength has to be built into the parts. Race engines are specifically “balanced” on specialized equipment. This combined with better alloys and manufacturing techniques (“billet” parts where a production engine uses cast parts) and shorter strokes allows racing engines to rev much higher than stock engines.

Mass increases with the cube of the RPM...  

Care to elaborate on this breakthrough in physics?

Yeah, that doesn’t sound right does it…Somewhere there is a formula for calculating the LOAD on pistons and connecting rods that uses their mass (weight), the engines design RPM and the stroke measurement…

Ah yes, the Pratt & Whitney R-4360…4 rows of seven cylinders, 28 cylinders all together, 4300 horsepower out of an engine that weighed around 3950 pounds. Overhaul required every 600 hours…That’s about every 90 days in commercial or military service…

Another example, the last of the steam locomotives, some of which used 8’ diameter drive wheels with huge counter-weights, to give 100mph speeds, needed these huge wheels to keep the reciprocating mass of the connecting rods, which weighed tons, under control…

On a related note:
I remember years ago in my internal engines class, we compared various aspects of an .049 engine (operating at 20,000 rpm) to an ocean steam-liner engine (where the piston was big enough to park a VW bug on, went up-n-down 2 stories, and maxed out at 175 rpm).

Ironically, the two engines had very similar feet/second piston speed.