Crossplane Crankshaft

These days this word “Crossplane Crankshaft” is a very common word in MotoGP. Well in racing what does the rider look for???? The rider looks forward for three things Right power, right torque and a properly setup suspension to transform it on the track. More power, more torque and no proper  suspension means there is no point in racing. A racing bike or a car is a combination of power and torque. Now consider power is given to the wheels when the driver demands it and how does he carrier it through out the race is based on the suspension. When the wheels are struck to the ground the driver gets the seventh sense of where exactly the grip is on the track…. So here lets discuss about the power factor.

So what Yamaha has done with an existing crankshaft, What makes the Yamaha M1 or the R1 now to get the maximum grip? Yamaha has developed a new generation of crankshaft called the “Crossplane crankshaft”. How does it help in getting the maximum grip,Well it changes the firing order of the engine just because the crankpin’s are offset between each other. Trust me this is not the only technology, there are several methods which is followed by Yamaha to get it such as YCCi(Yamaha Chip Controlled Intake), YCCT(Yamaha Chip Controlled Throttle) and the suspension set up etc.

Though this technology was present in the older generation “V” engines Yamaha gave life to this technology in their race engines which was driven by Rossi and Lorenzo. Recently Honda has also started using a similar technology in the new generation engines.

Crankshaft:

Well this is purely a mechanical part of the engine. According to me “ Its a part which converts the linear motion of the piston and connecting rod to the rotary motion of the flywheel ” and they also transmit the energy to the flywheel. Crankpin helps in achieving the desired firing order of the engine.

 

 

                                                                              A four cylinder engine Crankshaft 

We shall talk about the parts of the crankshaft later, First lets see what is Firing order,

Firing Order:

Firing order is nothing but deciding which cylinder should fire when based upon the engine configuration and design of forces that’s operating with the engine. A normal four stroke four cylinder engine will have a firing order of 1-3-2-4 or 1-4-3-2. There were some old engines existed with 1-2-3-4 firing order also. So, what is the common factor in deciding the firing order of the engine???? Well like I explained earlier it’s all based on the balancing of the primary and the secondary forces of the engine and provided it (the firing order) defines the application of the engine. A constant speed engine which is used in electrical application may have a very low forces acting on them because of a constant running speed and there is no other running or resisting medium such as roads, aerodynamic effects etc.

Parts of a crankshaft:

• Flywheel mounting flange is the place where the flywheel gets mounted to the crankshaft. They will have a “Dowel” for the easy location the holes where the bolts come and fit. These dowel helps in locating the holes through which there is no misalignment of the bolts.
• Crankpin oil hole is which the lubricating oil flows to the big end of the connecting rod. When the hole in the bearing and the hole in the crankpin are matched, lubricating oil flows from the crankshaft to the connecting rod.
• Counterweight’s are normally used to reduce the vibrations that are occurred when the piston moves in the cylinder. They also provide rigidity to the crankshaft.
• Main journals is the place where the connecting rod comes and fits. If its an Inline engine one connecting rod fits in the main journal and & in some V-Engines two connecting rods are also seen. A four cylinder inline engine will have five main journals.
• Crankweb connect the main journal to the crankpin’s.
• There are spaces at the end of the crankshaft from which the drive to the cam (through a belt drive) and the vibration dampers are taken.

Well the crosspane crankshaft’s design is purely based on changing the angles of the crankpin which will alter the firing angle respectively.

Crossplane Crankshaft:

Well how different is the crossplane crankshaft from others. What makes it so special?

Now, consider a normal four cylinder engine with an firing order of 1-3-4-2 and it runs on a four stroke cycle. In that case the engine will have a useful stroke (Power stroke) every 180 degrees of the crankshaft in each cylinders. That is first cylinder will fire at 180deg and then the next power stroke comes in the third cylinder after 180degrees. Thus they are interlinked and designed flat that for every 180degrees there is a power stroke or I can they are designed with an “Even Firing Order”.

Yamaha’s crossplane crankshaft has a “un-even firing order” that is the engine will not have firing order for every 180degrees. The crankshaft is designed to have an firing order of 270-180-90-180 degrees. That is the first cylinder will fire at 270degrees of crankshaft and the third followed by next 180degrees of crankshaft. This is the change what they made in the crankshaft to get an even flow of torque in the wheels. But why was this necessary when all the engines run on a even firing order in the world?

 

                        

                            Crossplane Crankshaft                                                                                   Conventional Crankshaft

  As you can see in the image in the crossplane crankshaft the pistons move individually and in the conventional two pistons move up and down because of the even crank design.

 

How torque is produced in a Engine?

Torque is nothing but the maximum force that the engine can produce within the specified range and it should not be mistaken for the Work output(Power) of the engine. Well coming on to the topic how the torque is produced, When the combustion in the engine happens due to mixture of air and fuel an external agent aids the firing then the fuel and the air burns and produces energy. This energy is passed on to the crankshaft, Flywheel, and to the gear box finally to the wheels. When the wheels rotate they produce torque or power from the requirement of the driver.

In simpler language I can say that,

Torque:- How fast you can hit the wall…

Power:- How long you can hit the wall…

Ok the torque in the engine is produced. Now for the next few meaningful definition lets jump in to bit of Engineering,

If the flywheel is very light and all other moving parts are light they spin faster which successively can produce more torque.

But on the other side if there is too much of weight within the moving parts they Spin slow because of the resistance what they have within themselves just because they are heavier leading to less torque produced. Well this is not the only factor there are many factors which limit the torque produced in the engine such as fuel limitations and engine’s other operating factors such as the maximum combustion pressure that the engine can hold etc.

So what ever be the part that rotates or moves simply subjected to any motion they produce certain amount of Inertia which is required to keep them performing the same action. Similarly in an engine to keep them rotating there should me a minimum amount of force which keeps the moving parts spinning. This is done by the combustion of the fuel.

Fine in that case I might as well have a lighter moving parts which requires low inertia. Why is crossplane crankshaft so special? Why such a complicated design is needed?

 What exactly Happens?

When the driver sends his demand through the throttle to the engine,what he gets as a output is the torque and power from the engine. The torque what he gets is the combination of “Combustion” and “Inertia” torque. Combustion torque is experienced by the burning of fuel and air mixture which is pretty straight forward. The inertia torque which the driver may not feel but what is gets is mainly due to the Inertia i.e. due to the motion of the moving parts in an engine.

Overall Torque = Combustion torque + Inertia Torque

 The above image explains how the Combustion,Inertial and Composite torque are produced.

Only the combustion torque is felt by the driver but the inertia torque is not felt by the driver. Which means the combustion torque is controlled by the throttle from the driver and inertia torque is purely based on the speed of the crankshaft. Now due to which when the driver always feels that there is a slight delay in the flow of power to the wheels. This is purely because of the inertia torque produced in the engine. What the driver wants is the combustion torque but he gets in the combination of the combustion and inertia torque. The reason why people are working to eliminate

 

 Four Stroke Working Cycle

Now why is this inertia torque is so critical… What makes it so complicated… Well when the crank moves up and down it travels faster during their top and bottom movement. But they are travel very slow when they are in 90 and 270 degrees, its purely because @ 90 and 270 degrees the whole of the piston and the connecting rod weight is pushing the counter weight to move further. Its extremely opposite in the other side to push the counter weight down the engine has to give a little effort and the rest is taken care by the gravity. So if you consider at the 90 and 270 degree positions its purely because of the inertia of the moving parts. So this delay is proportional to the speed of the  crankshaft. At higher speeds they are very critical. What exactly happens is the driver wants the combustion torque and since the crank train is spinning faster the output will be the combination of the inertia and the combustion torque. So the driver gets a unpleasant surprise when he revs the engine. This unpleasant surprise can be felt in higher rpm’s especially in motorsport engine when they rev for more than 19000rpm.

 

 Inertial torque goes proportional to the speed and the combustion torque goes proportional with the drivers demand

So what did the Yamaha do to keep the inertia torque low? Well if you see the picture of the normal crankshaft below two cylinders either cylinder 1 or 4 or cylinder 2 and 3 based on the firing order. Since they move together the crank web and the counter weight match the other cylinder movement. In this case it’s really hard for the engine to reduce the inertia torque because of the movement of the other cylinder and they strain a bit mainly due to the all the counterweight force acting on one side.

                                

                                  

                                Normal Crankshaft                                                                                        Crossplane Crankshaft

Yamaha made the position of the other cylinder’s counter weight exactly to the opposite side. Refer to the picture below (Crossplane crankshaft) so if the 3rd cylinder movement is slow in 90 degree position the other cylinder counter weight will be exactly opposite to the other so that it forces the 3rd cylinder movement to revolve faster. Due to this small shift in the position of the crankpins Yamaha was completely able to avoid the Inertia torque.  

                    

        Due to the new alignment the Inertial torque is completely avoided                                         Honda’s global crankshaft design

 

Interestingly Honda also adopted this new technology in their crank designs for the 700cc engine. Though these kind of crankshafts are higher in terms of the manufacturing cost when compared to the older one since their advantages are on the higher side every manufacturer particularly in the bike segment are coming forward for this old technology which has been existing for a long time. Yamaha claims that the bike is much better in high speed corners than the old one. Surprisingly Yamaha claims there is a significant increase in the tire life also. But Yamaha and Honda has used a secondary balancer shaft to reduce the secondary vibrations of the engine. May be in the near future we can see all cars even come with an un-even firing order to increase the drivability mainly for smaller engines.

The following videos explain the working of the crossplane crankshaft system

SKYACTIV Engine from Mazda

Mazda very recently developed a new technology for their Gasoline and Diesel engines known as SKYACTIV, which recently changed the myth of all internal combustion engines. Well myself, being a devotee of Gasoline engines lets see what difference these engine make when compared to the existing ones.

Most of the books or engineering myth states that petrol engines can achieve an compression ratio of 10.5:1 and these days a normal street car have an average compression ratio of 10,Unless and until the engines are  designed for a specific purpose like racing or what so ever.

But the engineers in Mazda proved that these myths are wrong which eventually changed the mythoi of internal combustion engines. Yes they are really proud enough in saying that they have made the world’s first mass produced gasoline engine which achieves the compression ratio of 14:1 which is really the highest among the mass produced gasoline engines.

According to thermodynamics increase in compression ratio results in increasing  thermal efficiency of the engine , which means chances of engine exposing to higher  temperature’s are very high. But increase in thermal efficiency has not only got the adverse effects but also the some better upshots in terms of engine power etc. Smaller capacity race engine with high compression ratio results in better power output. Most critical parts like the Valve face, Piston crown, spark plugs, cylinder wall and the Piston rings are exposed too much higher operating temperatures than the normal engines. This is mainly due to the combustion temperatures or commonly called as in cylinder temperature that are very high in the engine. It increases to an extent that the homogenous mixture starts to ignite them self before the spark actually comes and this phenomena is called as KNOCKING.

KNOCKING:

As you can see in the Image, in normal combustion a homogenous mixture of air and fuel at definite proportions based on the drivers demand is sprayed into the combustion chamber. Just before the piston hits the TDC there is a spark from the spark plug which travels almost to the end of the cylinder wall to make sure that the mixture is completely burnt for producing energy.

                            Difference between Normal combustion, Knocking and Pre-Ignition

During knocking scenario before the spark reaches the end of the cylinder wall due to the increase in cylinder temperature there is an external spark aid which travels opposite to the flame front and when they both collide they produce a noise which is similar to the sound of “knocking”. There are other factors that also aid in knocking such as poor air fuel mixture etc.

Pre – Ignition is even worse than these where the spark comes much before the AF Mixture is sprayed.

Now in modern engines special sensors called “Knock Sensors” are used to detect the knocking in the engine. When ever the knocking is detected the engine management system either closes the butterfly valve (through which the air comes) automatically or retards the ignition timing so that there is a very less load observed. These sensors are located in the engine block or near the cylinder head.

Location of Knock Sensor

If increasing the compression ratio leads to such an contrary effects how did the Mazda engineers achieve it. The following are the techs that are used by Mazda to achieve it….. 

1. Variable Valve Timing
2. 4-2-1 Motorsport Inspired Exhaust System
3. High Compression Engine with an Special design in the Piston
4. Last but not the least the Direct Gasoline Injection

4-2-1 Motorsport Inspired Exhaust System:

Normally this kind of exhaust system initially was used in racing engines where each and every RPM is important to get the best out of the engine. 4-2-1 Exhaust system a.k.a 4-2-1 headers or tri-y to may race enthusiastic is mainly known for better  low end  and mid range responses in racing engines. Mazda applied the same in the skyactiv engines also. Engineering says that by joining various cylinder tubes before the collector multiple pressure waves can be created. Consider a four cylinder engine from which all the four exhaust pipes are joined in to two collectors based on the firing order. 

A normal four cylinder engine has a firing order of 1-3-4-2, the exhaust pipes from 1-3 are joined to a common collector so that when the first cylinder begins their exhaust stroke (A positive pulse is produced)  the gases takes considerable amount of time to meet the collector and they are also pushed back to the 3rd cylinder (Can also be called as Negative Pulse) ,when the exhaust valve opens in the 3rd cylinder these pulses however hit in the right time to pull the residual gases from the other cylinder provided they are properly tuned.

In any cases the exhaust system plays a considerable role in the performance of the engine. Any exhaust system they work on the principle of resonance. The pipe diameter, length, collector angle its length everything plays a major role in the performance of the engine.

 

A custom made 4-2-1 racing header

High Compression Engine with an Special design in the Piston:

As discussed earlier high compression means higher expansion to heat, What is this compression ratio is all about and how it plays a major role in Skyactiv engine,

Compression Ratio:

They are nothing but the ratios to the gases are compressed when the piston moves from BDC to TDC.

Imagine an engine with an compression ratio of 10:1 which means there are 100cc air that can be absorbed during inlet stroke and they will be compressed to 10cc when the piston reaches the maximum point. This is called as compression ratio. The more this number the better the performance provided they are under the limits. For a gasoline street engines the typical comp ration will be around 10 to 12:1. Racing engines goes up to 14:1.

TO attain this there are several ways in a engine,

• Domed Piston
• Cylinder head of smaller combustion chamber area
• Thinner gasket
• Reducing the height of block and the cylinder head.

But Mazda uses a domed piston to increase the compression ratio of the engine. Normally one important factor when choosing the domed piston is the compression height. Compression height is nothing but the distance between the centre line of the piston pin (Gudgeon or even known as wrist pin) to the top of the piston.

Compression height of piston

Nowadays all manufacturers are either going for a flat or undercut (Dish) pistons in stock engines. Most of the piston which has domed head is either found in racing or high performance cars. Mazda uses a domed piston which has helped them in getting better compression ratio compared to other competitors.

SKYACTIV-G Piston

Difference among conventional and the skyactiv-G pistons

The difference is as seen, the conventional piston has got an undercut (Dish) and Mazda has gone for a dome shaped piston. The cut in the valves are to make sure that the valves doesn’t hit in the piston when they are at the maximum level. These kinds of engine are also known as interference engine. The cut is mainly to give an extra amount of lift to the valves which sends more amount of air fuel mixture. Moreover some say that these also help in maintaining the compression ratios.

They do have some disadvantages i.e. if anything goes wrong with the timing they collide with the valves and chances of getting valves bent are very high.

Wonder why there is a hole in the middle of the piston???????? Well I feel that in order to have stratified fuel mixture in the engine there is a cavity in the piston head. This might also help in better roll, squish and turbulence of gases present inside the engine.

Stratified charge?????? Wondering what again stratified charge means the density of the air fuel mixture changes when compared to the centre of the piston than to the cylinder walls. There is a very rich mixture which can near the spark plug which can initiate  combustion immediately and around the cylinder walls there is a mixture that is lean enough. They are commonly used in Gasoline Direct Injection engines where the amount of fuel in controlled rather controlling the amount of air. Yes Mazda uses gasoline direct injection in this engine.

Gasoline Direct Injection:

As the emission norms gets more rigid thought the world there is always a need of newer technologies that has to be adopted by the manufacturers. One such technology which was commonly accepted by manufacturers was Common rail diesel injection (CRDi) as known to many for diesel engines. Still for gasoline engines some manufacturers accept the MPFi (Multi Point Fuel Injection System) some move ahead with times and they started accepting Gasoline Direct Injection. The first on this league was Mitsubishi way back in the late 90’s.

A detailed explanation on the overall GDi system is discussed here…

What difference does GDi make? How different it’s when compared to other systems? The answer is given in the picture below,

As you can see in the earlier carburetor system both the air and the fuel are mixed and sent to the combustion chamber. There was a very limited control of the fuel which was sent. Moreover altitude compensation, Temperature compensation was very limited.

Then came the Port Fuel Injection in which there was precise control of the fuel which was metered either by sensing the manifold vacuum through MAP sensor or by sensing the mass of air that is entering. By sending these information's to the Engine Control Unit (ECU) the ECU reads its pre-determined or programmed map and sends the right amount of fuel that is required. Maintaining the engine under control was much easier with the PFI system. The engineers were able to meet the emission norms across the globe.

In the second stage of these PFI injection systems the entire system was bought under closed loop by adding one more sensor called Oxygen sensor. With the ECU was able to adjust the  fuel demand more precise.

Gasoline direct injection is little bit advanced than the PFI system. Rather than sending the mixture in the manifold the fuel (Gasoline) is injected in the combustion chamber directly like diesel injection. With technologies like this manufacturers are able to get the required power in smaller capacity engines itself. Thus they became more fuel efficient because of the various control systems in the engine electronics.

Working:

Any GDI system works on three modes

• Homogenous  mode
• Stratified Charge mode
• Homogenous  Lean mode

Homogenous Mode:

In this mode a  stoichiometric mixture is formed in the combustion chamber where the AFR of 14.7:1 is maintained or even richer  At this point the emission parameters are well under control. And as we know all modern engines are closed loop engines the AFR are well maintained within the limits through out the range. The fuel is injected when the piston is in the suction stroke. It takes enough time to mix with the considerable amount of air that is already available in the engine.  This mode is like any other port fuel injection system.

It’s in this stage where all the emission parameters of an SI engine such as CO,NOx and HC are under the limits.

 

Emission formation of an SI Engine

In the Image as you can see exactly @ 14.7:1 the exhaust emission’s re well in the control. Anything more say for example 13,12 HC and CO are higher. Leaner mixture results in increase of HC and NOx.Well it doesn’t mean that 14.7:1 is perfectly all right and why do we need some costly exhaust treatments like catalytic converter etc. It’s just because to keep the emission well in limits in for suiting different norms.

Stratified Mode:

This is where the GDI make their presence; these modes are normally used for light load running conditions or at constant speed or even low speeds where the amount of acceleration required is very low or constant. Under these cases the AFR can go up to 30:1 or even higher. The fuel is injected much before power stroke as a small amount of fuel is enough to make the combustion. This fuel which is sprayed should be as near as possible to the spark plug. But the disadvantage is that very lean mixture’s produce NOx emission because of the operating temperature conditions.

Therefore some aid is required to bring the cylinder temperature down. This is where the EGR comes to picture. The EGR (Exhaust Gas Recirculation) dilutes the temperature in the combustion chamber by sending some burnt gases in to the combustion chamber. When the temperature of the  charge in the combustion chamber are reduced the cylinder temperature is also reduced thus knocking can be avoided and NOx emission is also under control.

Not forget to mention that there is always a Wideband O2 sensor present in these engines as mixture can vary their ratio rapidly.

Wideband O2????????? Wonder What????? Well as mentioned earlier all port fuel and GDI engines run on closed loop i.e. the system is considerably monitored for giving their maximum efficiency. These O2 sensors tell ECU the exact amount of air present in the exhaust gases. The amount of Oxygen present in the  exhaust gases help the ECU to decide if the mixture that was sent is rich or lean.

There are two types of O2 sensor’s Wideband and Narrow band. All low cost engines will have narrow band sensors which can only tell the ECU if the fuel is rich or lean. Not how rich or lean.

Wideband sensors tell ECU the exact digit say 12.5 or even 17 etc in terms of voltage for which they are calibrated.

Homogenous Lean Mode:

This is something between Stratified and Homogenous mode. The AFR value will be around 18:1 etc which is little leaner not so as stratified. Based on the AFR the EGR can also be activated to reduce the in cylinder temperature. They are normally used in light cruising ranges where the engine load is not changed rapidly.

Well that is GDI which helps the Mazda SKYACTIV engines to get better performance and emission and much better efficiency also.

Variable Valve Timing:

Refer my previous post for better explanation on the VVT systems.

The following video explains the working of the Mazda SKYACTIV-G Engine

SKYACTIVE-G

More Video on GDi Engines