I was thinking last night about how mazda used a twinscrew supercharger to increase the efficiency of the Millenia – the Miller cycle engine. The briefest overview is that this engine uses a higher mechanical compression ratio, combined with the supercharger to pre-compress and cool the intake charge, and valve timing to control the amount of cylinder fill. For example, a theoretical Miller cycle engine could have a supercharger generating 7 psi of boost pressure, and adjust valve timing to close the intake valve when the piston is 50% of its way back to TDC on the compression stroke. the end result is there is slightly more air in the engine due to the ability to cool the air from the SC before it is compressed he rest of the way in the cylinder. The final temperature is also lower, allowing different ignition timing and higher compression pistons to be used which give the advantage of higher efficiency.
All that is well and good and completely understood by some smart engineers somewhere. I started thinking about the idea of the supercharger being a more efficient compressor than a piston in a cylinder. Immediately after that I thought about how roots ( positive displacement) superchargers kind of suck compared to centrifugal type or turbocharger with respect to efficiency. So The idea hit me, why did the Miller cycle do any compression at all with the pistones movement? I suspect because Mazda’s design target was to improve low engine load efficiency, but still have the capability of utilizing large amounts of power – i.e – how a typical american drives, one pedal always mashed, especially in stop and go bumper to bumper traffic. If there is no requirement to make ‘big’ power, we could compress small amounts of air to the same levels the piston sees at TDC under partial throttle pretty easily.. partial throttle is when you are crusing on the highway and want really good gas mileage. The engine is doing a lot of work to create vacuum by sucking the restricted amount of air the driver is allowing with the gas pedal controlled throttle plate. So sucking air past that throttle plate and generating 15 inches of vacuum when the piston is at bdc ( bottom dead center) and then the piston moving back upwards, recovering much of this work, and then compressing what is still left means the cruising cylinder pressure is pretty low. Lets use my imaginary engine that has 15″ vacuum – which is about 7 psi absolute pressure. This means that the cylinder has 7 psi in it at BDC, and since my imaginary engine is a 10:1 static compression ratio engine, it has 70+ psi at TDC. the pressure will be higher than 70 because the air will be quite hot. If the driver had to pass someone and floored it, all 14 psi atmospheric pressure could fill up that cylinder at BDC, and the piston would compress it to 140+ psi at TDC. In reality the pressure would be more like 180-220 psi due to the air heating effect due to inefficient compression.
So instead of sucking air past that throttle plate, what if the motor had an efficient compressor attached to it somehow ( turbo, centrifugal supercharger, twinscroll or roots blower – or multiple stages of compressors that we could engage and disengage somehow – a wastegate for the turbo, a clutch for the supercharger, or use a variable displacement pump and vary displacement from 0 to X as needed. We can size this so that it creates 70 psi air for us, intercool it, and then pass it to the cylinder via a re-engineered induction system when the piston is at or near TDC, and then use direct injection to add fuel, ad some spark, and go to town. This gets rid of the throttle plate for lower pumping losses, provides a cool dense intake charge that has no tendencies to ignite our fuel too early, allows us to increase compression ratio drastically for more efficiency, and by adjusting the pressure / volume of air put int the cylinder at TDC, allows us to regulate power easily.
Power regulation is the next topic of this engine – chances are the steady state pressure / flow numbers can be optimized quite a bit. If this new imaginary engine has an 18:1 or 25:1 compression ratio and a long stroke, less air and fuel would need to be consumed to generate the same power as the normal 10:1 engine sucking past the throttle plate, and heating the air up during the compression pat of the cycle. So for this example, say to cruise on the highway at 10hp it needs 13 cfm or .9 lb/min of air. Our pump should be the most efficient pump possible to generate this amount of air at whatever the required pressure ended up being. What happens when the driver needs to pass someone then? Obviously we need more air, but how much more air? With that high 18:1 compression ratio, and no vacuum pumping losses, and more efficient intake charge compression, it will not be as much air as a typical engine, but it will be substantially more than the 13cfm used to cruise. Say the driver needs 100 hp of power to pass – like 130 cfm or 9 lb/min worth of air. Obviously this is a lot more volume than that optimized pump setup could supply, so some additional air must come from somewhere… how about a high pressure tank under the car. A typical welding tank is a 330 cu/ft tank -almost 3 min at 100hp level consumption rates. This wont work for a racecar, but to commute to and from work, it should… Pro benefit is all this compression can be done at home using electricity – cheaper than gas, cleaner than gas for most areas of the country. Seems like I have heard a lot about the idea of a plug in hybrid lately.. Does this qualify? How about if we alter our compression system so it is flexible enough to refill this tank as you drive by consuming power during braking? Did we just create a hybrid? Will the steady state pump even have to run most of the time? If you are cruising at 13 cfm from a 330foot tank you can go 25 min without running out, and that does not take into account any regen you get when braking.. Maybe the compressor can now be a much higher pressure pump that runs on a low duty cycle for cruising to keep the tank at a minimum level, and is completely off for higher power operation. All of this could be computer controlled to the extent of telling the car the trip you are about to take and where it can be recharged ahead of time. If google can tell me how long it will take me to get home in current traffic, telling the car I am going to work and back home should allow it to optimize for utilization of that full tank each morning by adjusting the duty cycle of the on vehicle pump to allow the tank to run down to some safe reserve amount over the course of the two trips. Obviously, if something unforseen arises, the onboard pump can just run more to keep the tank above that safe reserve level.
Imagine if we had some sort of device that had a GPS in it, that knew where we were going, and could remember the route from the previous times we took that route – elevation changes, speed, braking, merge acceleration etc. – and optimize the pump / tank system to consume more when we are within a mile from a long downhill off ramp when it would regenerate itself via braking, but to run more when it knew that there was a long grade ahead that it could pre-pump the tank for the entire grade… Computers – freaking magic. Pocket sized computers with many processors, GPS + wifi communication etc but our cars are still so dumb.
Another major benefit to this idea is the power to weight ratio initially looks like it will be terrible – adding in a big tank ( scuba carbon fiber types might not weigh much compared to my stel welding tanks), and adding in an external compressor. A positive is the intake manifold and throttle body can go away entirely. Supplying air from a high pressure source will be more like braided steel teflon core or hydraulic lines + some solenoids than thick aluminum castings. The head itself can be smaller as the intake port is useless area, we need as close to direct cylinder injection o the air as possible. The intake port is a dead concept for this motor. The intake valve has to be re-imagined as more of a solenoid with a much shorter on / off time / duration, and smaller area requirements. We might be able to go much larger for exhaust valves as there will be more space for them, assuming they can stay cool enough.
And the best for last – we just got rid of the compression stroke from this motor right? The intake event is a high pressure gas being released into the cylinder at TDC, so we also got rid of the intake stroke.. How many strokes are left? Power and exhaust? 2 stroke engines have double the number of power strokes as a 4 stroke motor.. this motor just doubled its hp/displacement numbers. Running at 18+ :1 compression ratio, it is going to be making a lot more power due to its higher efficiency also… And then the only thing regulating more power is the ‘on time’ of the air supply solenoid and the correlate direct injection fuel supply.. The knock limits we know and understand now are for engines that have a hot intake charge… we limit compression ratio to keep this temperature under control. entirely new knock limits will have to be learned on this motor, and it might be that we can supply it with 300 psi of air blowing way past what a typical otto cycle engine could handle.. 300 psi sounds like a lot, but cold compression tester numbers on some 4cyl motors are already in the 250+ range without the benefit of the entire intake charge being at or even below ambient temp.
In fact, if the supply tank is at 3000 psi, supplying 10-300 psi air is going to be some rapidly cooled via expansion air, and probably necessary to go through a heat exchanger with any gas being compressed in order to keep it above minimum temps that gasoline / air mixtures burn well at!
Next steps for this Idea are to research if anyone has already done it – like pretty much every awesome Idea I come up with, I find patents already exist. If so, add it to my list of “almost first” ideas. If not, then start doing some math and other research
- solid numbers on air requirements for 10 hp and 100 hp when the compression ratio is ridiculously high and the intake charge is ridiculously cool, ( examples I used came from online calculator for a typical motor).
- compression efficiencies of a piston compressor vs a screw, roots, impeller etc.
- how difficult would it be to convert an existing engine