Spitfire XIVs equipped with nitrogen drop tanks

Shortround6 · Jul 10, 2015

There have been some tremendous advances in what is possible with the engines in the last 70 years but one thing often left out of these car vs airplane comparisons is weight. The other thing is durability.

A modern engine is much better than an old airplane engine in many ways but without factoring in both of those things it makes the modern car engine look somewhat better than is really the case.

AS for power per liter the British managed 155.9 hp per liter in 1936/37 with the Austin 7 race race engine. A 750cc 4 cylinder that could make 116hp at 7600rpm running on a rather exotic blend of fuel at an intake pressure of 2.66Atm. It also weighed 260lbs or 2.24lbs per HP. not so good for an airplane engine. The Germans managed 162.1h per liter in 1939 with the Mercedes Benz Formula I engine. 2,962CCs managed 480hp at 7500rpm at 2.31Atm. Also a rather exotic fuel blend and the engine weighed 603lbs for a power to weight ratio of 1.26lbs per hp.

Modern engines use much better metals to allow higher rpm which allows for higher airflow per minute. The better metals also allow for lighter weight and greater durability. However if you blow up your VW engine because you were running it flat out for a few hours on end on the autoban you get to pull over and wait for the tow truck. There wasn't a sky tow service in business during WW II Pulling up to a near by cloud and waiting 20 minutes for the tow plane wasn't an option.

Koopernic · Jul 10, 2015

Shortround6 said:
However if you blow up your VW engine because you were running it flat out for a few hours on end on the autoban you get to pull over and wait for the tow truck. There wasn't a sky tow service in business during WW II Pulling up to a near by cloud and waiting 20 minutes for the tow plane wasn't an option.

I don't know if you've driven on an autobahn but you can definitely drive flat out in a VW for 4 hours, Ive done it and its completely normal for any 20 something who can only afford a hatch for a few years. Flat out pedal to the metal. German cars don't mind it. Nevertheless some bad experiences with such runs lead to the development of the MON and perhaps a penchant for German cars to recommend higher octane levels.

Every weekend in the early 1990 I drove from around Frankfurt to Near Lake Constance to my English girlfriend who lived on the Swiss/German border. The 1992 Golf CL had a 1.6L carburated engine, the registration papers stated it had a power of 75ps and a tops speed on a standard flat road of 157kmh, that speed estimate was pretty accurate could reach 190km in a tail wind or obviously down hill.

It was definitely one of the slower cars on the road and I yearned for the power needed to accelerate past a slower driver who was driving inconsistently etc. I remember a Lamborghini Diablo shooting past then cutting in front of me to cut of the autobahn exit, it seemed he just flicked 30 degrees at 130km without a skid and I saw the car in side profile. Latter that weekend I saw the car pottering on the lake side, me on a push bike. Noteworthy was the fact that the Lamborghini exhaust note, though quiet, resonates with the human chest cavity to leave a very powerful impression.

Two decades latter I was back, in an 2.0L SLK Compressor, for a few weeks. It sat nicely at whatever speed I wanted to go, but you definitely wanted more power for overtaking manoeuvres. Again there was a guy with a better car. I remember struggling to overtake in the middle lane, while knuckles white from the speed that I'm not used to, while a guy in a late model 911 serenely sailed past me, the top down, the air dam up so that his girl friends pony tail didn't move while she sunned her legs on the dashboard nonchalantly bantering with her beau.

These engines don't bust. I believe the MON came out of problems encountered with fuels on these runs.

Koopernic · Jul 10, 2015

Shortround6 said:
I wonder how much power it makes at 3000rpm?

It probably doesn't matter, rapid shifting 7 speed gearboxes are almost the norm and 8,continiously variable are normal as well.

The 1.4L TSi engine is both mechanically supercharged and turbocharged. In its latest incarnation is can shut down cylinder to save fuel. This engine is what VW offers on its lower models though it is a fine engine in terms of response and torque. Its likely that promises by the auto industry to Governments to reduce co2 tail pipe emissions make it essential to sell large numbers of such cars.

post edit, the latest version of the engine no longer uses a mechanical supercharger.

fastmongrel said:
You take the VW Tiguan above 10,000ft and it would struggle to pull the skin off a Rice Pudding. In fact the ECU would probably go into limp mode or even shut the engine down completely.

I doubt it, cars such as these meed to work at such altitudes in chile, peru. I'd say the boost control would compensate nicely I only made note of The Tiguan's engine to emphasise who within reasonable limits the Griffon and Merlin were.

pbehn · Jul 10, 2015

The guys on the autobahns who do go flat out all day are the ones in vans.

pbehn · Jul 10, 2015

The guys on the autobahns who do go flat out all day are the ones in vans.

Koopernic · Jul 10, 2015

Aggressive aren't they. Time is money I suppose.

yulzari · Jul 11, 2015

Koopernic said:
My expectation is this:

The RAF was experimenting with Nitrous Oxide on the Mk XIV Spitfire. The note Mike William's posted indicate that there was interest in using NOX not only above the aircrafts full throttle height but at low altitude. The problem being that improvements above 150PN fuel were unlikely and that seemingly the Griffon could take a much higher PN fuel. NOX was seen as a way of increasing the knock rating of the mixture.

In Luftwaffe usage GM-1 (Goering Mixture 1, cryogenic nitrous oxide) was used above the engines full throttle height (where the engine could take the full unthrottled throttled force of the supercharger) whereas MW50 (water methanol) injection was applied below the engines full throttle height. Late war Me 109K4, Fw 190A9, Fw 190D9 had a tail tank of around 115-130 litres that was to carry either extra fuel (either B4 or C3), GM-1 or MW50 depending on mission and availability. Only production/delivery issues prevented the delivery of the plumbing though two Fw 190D9 were flown, in service, with GM-1 in the tanks.

There is nothing to prevent us from using nitrous oxide at low altitude so long as the same power limits are observed to prevent bearing damage and to not exceed thermal limits as were imposed on use of MW50 (ADI injection in allied terminology). It was just more convenient and cheaper to use MW50 for low altitude and GM-1/Nitrous for high altitude. The DB605DB/DC went from about 1450hp on B4 to 1800hp on B4+MW50. So long as the nitrous was used judiciously and max power kept below the 1800hp certified for MW50 the engine should be fine. I rather suspect nitrous was kinder and less corrosive and as the RAF papers noted it relieved the supercharger of having to supply the power for over boosting.

I do not see it a problem to apply Nitrous Oxide to the Spitfire XIV. There was a tail tank after the pilots seat I believe it was 44 gallons and that it was considered only suitable for ferrying as it upset the centre of gravity too much for good handing. 44 gallons of fuel weights about 130kg or 285 pounds. That's more than the 160lbs night fighter Mosquito were using. I'm sure that the Mk XIV would have coped with less than half that amount of nitrous, say 140lbs with little effect on handling. The Night fighter mosquitos were getting results from only 160lbs.

The Griffon clearly still had reserve and as was noted Nitrous Oxide could be used to relieve the power draw on the supercharger substantially and prevent pre ignition.

The RAF had the problem of potentially having to deal with flying bombs at low altitude and to get an intercept on say an Ar 234 reconnaissance jet. In such a situation the nitrous could both dramatically increase climb rate (and burning of any handling issues during medium altitude and speed.)

The Ta 152H had both M50 and Nitrous and could use them concurrently. Obviously since the RAF had a 150PN fuel they could more easily avoid water injection. They probably desired 160PN fuel but that was unlikely to be made available in quantity.

Below is the 2015 Volkswaggon Tiguan people mover used by moms to drop of children at school and go shopping at supermarkets. It's 1.4L TSi engine has a specific output of 100hp/Litre on 95 (or less) RON with lean mixture thus handsomely exceeding the performance of the Merlin or Griffon. That's equal to the Merlin producing 2600hp on 95 octane.

View attachment 296305

FWIW on @900bhp on a pressure charged 1.6 litre engine for F1 a Merlin would give @15,000bhp.

kool kitty89 · Jul 11, 2015

It shouldn't really need pointing out, but raw power:swept volume ratio of an engine is a fairly limited statistic to compare by itself.

Shortround6 · Jul 11, 2015

It works pretty well if you are comparing the same class and age of engine. Like 1950s pushrod car engine to another 1950s pushrod car engine. tends to fall apart comparing motorcycle engines to car engines let alone airplane engines even in years close to each other.

Spitfire XIVs equipped with nitrogen drop tanks

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