Don Terrill’s Speed-Talk

Only a bit OT, but that device, able to track pressure versus time versus crank angle, would answer a long-standing question of mine. "Is there a noticeable difference in pressure/time/angle of in a flat top piston coming up to a smaller/deeper chamber with a large squish area, versus a more modern shallow chamber with a dished piston?"

David Redszus wrote: ↑Mon Dec 10, 2018 12:44 pm Having faced the question of determining true compression pressure many times over the years, we have found a solution.

We have built an adapter for a pressure sensor that screws into a spark plug hole of a running engine with the pressure signal fed into a data logger. We can now see that compression pressure (both max and min) varies with speed, throttle position and induction flow.

We can also compare multiple cylinder compression pressures at a range of engine speeds.

Since compression pressure contributes to chamber temperature, resulting in variable ignition points, flame propagation and peak pressure locations.

If anyone is interested in more information about this device, let me know.

Interesting......So if this adapter screws in to the spark plug hole, where does the spark plug go in relation to this adapter? How do the work together?

PackardV8 wrote: ↑Mon Dec 10, 2018 1:37 pm Only a bit OT, but that device, able to track pressure versus time versus crank angle, would answer a long-standing question of mine. "Is there a noticeable difference in pressure/time/angle of in a flat top piston coming up to a smaller/deeper chamber with a large squish area, versus a more modern shallow chamber with a dished piston?"

Will await David's reply, but since such a system cannot reliably react to turbulence, squish jets, etc., I think that in addition to CR, IVC and gas and metal temps., only the rate of volume change, which is independent of chamber shape, comes into it.

<EDIT> This is based on the assumption that the cylinder in question is being motored by external power or the remaining cylinders. If it's firing, there are of course far more factors at work.

David Redszus wrote: ↑Mon Dec 10, 2018 12:44 pm Having faced the question of determining true compression pressure many times over the years, we have found a solution.

We have built an adapter for a pressure sensor that screws into a spark plug hole of a running engine with the pressure signal fed into a data logger. We can now see that compression pressure (both max and min) varies with speed, throttle position and induction flow.

We can also compare multiple cylinder compression pressures at a range of engine speeds.

Since compression pressure contributes to chamber temperature, resulting in variable ignition points, flame propagation and peak pressure locations.

If anyone is interested in more information about this device, let me know.

With a spark plug in it or turning the cylinder into a dead hole?

Pico sells ready-made pressure transducers.

Interesting......So if this adapter screws in to the spark plug hole, where does the spark plug go in relation to this adapter? How do the work together?

Since the adapter replaces the spark plug, we are measuring a motored cylinder, presenting compression pressure, not combustion pressure; which is determined by the true (dynamic) compression ratio.

It will, however, also measure minimum cylinder pressure which results from the induction stroke. We can also compare cylinders, with and without, the induction system attached to the head. The effect of partial throttle can also be evaluated.

Remember that the maximum compression pressure will be a function of minimum cylinder pressure (usually at BDC), IVC angle, and inlet pressure at IVC.

With a little bit of help from our friends in the math dept, we could determine compression temperature at any crank angle.

Many years ago I ran a simple experiment to compare airflow through motored cylinders vs. running ones. With a (instrumented) stock exhaust and engine TPI (batch-fired) 1990 Camaro, I compared intake mass flow at steady speed WOT @ 5,000 RPM with and without the fuse pulled on one bank of injectors.
With all eight firing, the mass flow increased ~ 4-5%.

MadBill wrote: ↑Mon Dec 10, 2018 4:09 pm Many years ago I ran a simple experiment to compare airflow through motored cylinders vs. running ones. With a (instrumented) stock exhaust and engine TPI (batch-fired) 1990 Camaro, I compared intake mass flow at steady speed WOT @ 5,000 RPM with and without the fuse pulled on one bank of injectors.
With all eight firing, the mass flow increased ~ 4-5%.

Thats kinda where i was going with the question as to having sensor and spark plug in the hole and it running on said cylinder. Exhaust and scavenging has an effect on intake flow and that is what contributed to your 4-5% correct? If so his readings with his sensor in a dead hole are more like cranking compression only at higher speeds due to the engine running correct?

Right, the running engine moved ~ 5% more air. The difference would be quite dependent on degree of cam/exhaust tuning vs. targeted RPM of same and the test RPM. For example, a highly-tuned engine running at its design RPM might pull 15% more air and yet at some 'worst tune' RPM, 5% less, but conceivably a farm tractor would be near zero throughout..

MadBill wrote: ↑Mon Dec 10, 2018 4:49 pm Right, the running engine moved ~ 5% more air. The difference would be quite dependent on degree of cam/exhaust tuning vs. targeted RPM of same and the test RPM. For example, a highly-tuned engine running at its design RPM might pull 15% more air and yet at some 'worst tune' RPM, 5% less, but conceivably a farm tractor would be near zero throughout..

So im seeing a similarity in the four corner idle mixture screws and adjusting for max vacuum. Same principle as what you are saying here correct?

David:

What would be the cost of this spark plug substitute pressure sensor be and the related equipment and devices to read the measurements and record them? Sounds expensive.

splitdecision71 wrote: ↑Mon Dec 10, 2018 5:13 pm

MadBill wrote: ↑Mon Dec 10, 2018 4:49 pm Right, the running engine moved ~ 5% more air. The difference would be quite dependent on degree of cam/exhaust tuning vs. targeted RPM of same and the test RPM. For example, a highly-tuned engine running at its design RPM might pull 15% more air and yet at some 'worst tune' RPM, 5% less, but conceivably a farm tractor would be near zero throughout..

So im seeing a similarity in the four corner idle mixture screws and adjusting for max vacuum. Same principle as what you are saying here correct?

Ah, no. In my experiment we were comparing the engine's efficiency as an air pump with and without the assist of exhaust pressure wave tuning. Optimizing AFR and other aspects of the running engine is an entirely different proposition.

Newold1 wrote: ↑Mon Dec 10, 2018 5:44 pm David:

What would be the cost of this spark plug substitute pressure sensor be and the related equipment and devices to read the measurements and record them? Sounds expensive.

If budget was not limited, I would simply purchase an engine combustion pressure measurement system from TFX, and use it to measure inlet pressure, combustion pressure, compression pressure, and exhaust pressure. But in the real world, budgets are limited and alternatives must be found. And it must be affordable to most engine builders and racers.

With that in mind, I'm working on the details of a system that will measure and record: throttle position, rpm, compression pressure, inlet air temp, and exhaust back pressure, as well as lambda,if desired. That would include the necessary analysis software and tech support.

Compression pressure (and exhaust back pressure) measurement poses some problems since the air is pulsed at moderately high frequencies.

Minimum cylinder pressure measurement, as well as maximum cylinder pressure measurement for turbo applications, at various throttle positions and engine rpm is also difficult to obtain.

Tomorrow, I will be testing the system on an EFI vehicle with ECU and later on a turbocharged race engine.

I'll post my findings and progress. Any suggestions or ideas would be welcome.

Pretty sure Clint (Nitro2) can get you in the door with in cylinder only for $6k or so. Obviously more sensors required more cost.

In the grand scheme of racing automobiles (or bikes) it’s actually cheap. The problem is it’s just measures something. The real cost is in learning what it is showing you and how to take advantage of it. It requires a desire to have an open mind as well as the ability to think beyond your own inherent biases. It also will take a lot of time and effort. A lot of people are lazy and just stick to what they know, , and as Clint has also pointed out, is probably the bigger deal breaker than costs.

Plus, who has the clients for that? It would benefit a heads up racer or a drag / circle / road race builder who has front running customers and where the rules are somewhat limited. And of those clients, a racer who actually wants to do R&D as opposed to the same old recipe.

For a sensor kit only it's cheap. How much are replacement sensors ?

MadBill wrote: ↑Mon Dec 10, 2018 7:02 pm

splitdecision71 wrote: ↑Mon Dec 10, 2018 5:13 pm

MadBill wrote: ↑Mon Dec 10, 2018 4:49 pm Right, the running engine moved ~ 5% more air. The difference would be quite dependent on degree of cam/exhaust tuning vs. targeted RPM of same and the test RPM. For example, a highly-tuned engine running at its design RPM might pull 15% more air and yet at some 'worst tune' RPM, 5% less, but conceivably a farm tractor would be near zero throughout..

So im seeing a similarity in the four corner idle mixture screws and adjusting for max vacuum. Same principle as what you are saying here correct?

Ah, no. In my experiment we were comparing the engine's efficiency as an air pump with and without the assist of exhaust pressure wave tuning. Optimizing AFR and other aspects of the running engine is an entirely different proposition.

ok, thank you!